Pilots for mimo communication systems

ABSTRACT

Pilots suitable for use in MIMO systems and capable of supporting various functions are described. The various types of pilot include—a beacon pilot, a MIMO pilot, a steered reference or steered pilot, and a carrier pilot. The beacon pilot is transmitted from all transmit antennas and may be used for timing and frequency acquisition. The MIMO pilot is transmitted from all transmit antennas but is covered with different orthogonal codes assigned to the transmit antennas. The MIMO pilot may be used for channel estimation. The steered reference is transmitted on specific eigenmodes of a MIMO channel and is user terminal specific. The steered reference may be used for channel estimation. The carrier pilot may be transmitted on designated subbands/antennas and may be used for phase tracking of a carrier signal. Various pilot transmission schemes may be devised based on different combinations of these various types of pilot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/155,916, entitled “PILOTS FOR MIMO COMMUNICATION SYSTEM”, filed onJun. 8, 2011, which was a continuation of U.S. patent application Ser.No. 10/610,446, entitled “PILOTS FOR MIMO COMMUNICATION SYSTEM”, filedon Jun. 30, 2003, now U.S. Pat. No. 7,986,742, which claimed the benefitof provisional U.S. Application Ser. No. 60/421,309, entitled “MIMO WLANSYSTEM,” filed on Oct. 25, 2002, Ser. No. 60/421,462, entitled “CHANNELCALIBRATION FOR A TIME DIVISION DUPLEXED COMMUNICATION SYSTEM,” filed onOct. 25, 2002, Ser. No. 60/421,428, entitled “CHANNEL ESTIMATION ANDSPATIAL PROCESSING FOR TDD MIMO SYSTEMS,” filed on Oct. 25, 2002, Ser.No. 60/432,617, entitled “PILOTS FOR MIMO COMMUNICATION SYSTEMS”, filedDec. 10, 2002, and Ser. No. 60/438,601, entitled “PILOTS FOR MIMOCOMMUNICATION SYSTEMS,” filed on Jan. 7, 2003, all of which are assignedto the assignee of the present application and incorporated herein byreference in their entirety for all purposes.

BACKGROUND

I. Field

The present invention relates generally to data communication, and morespecifically to pilots suitable for use in multiple-inputmultiple-output (MIMO) communication systems.

II. Background

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission. A MIMO channel formed bythe N_(T) transmit and N_(R) receive antennas may be decomposed intoN_(S) independent channels, which are also referred to as eigenmodes,where N_(S)≦min{N_(T), N_(R)}. Each of the N_(S) independent channelscorresponds to a dimension. The MIMO system can provide improvedperformance (e.g., increased transmission capacity and/or greaterreliability) if the additional dimensionalities created by the multipletransmit and receive antennas are utilized.

In a wireless communication system, data to be transmitted is firstmodulated onto a radio frequency (RF) carrier signal to generate an RFmodulated signal that is more suitable for transmission over a wirelesschannel. For a MIMO system, up to N_(T) RF modulated signals may begenerated and transmitted simultaneously from the N_(T) transmitantennas. The transmitted RF modulated signals may reach the N_(R)receive antennas via a number of propagation paths in the wirelesschannel. The characteristics of the propagation paths typically varyover time due to a number of factors such as, for example, fading,multipath, and external interference. Consequently, the transmitted RFmodulated signals may experience different channel conditions (e.g.,different fading and multipath effects) and may be associated withdifferent complex gains and signal-to-noise ratios (SNRs).

To achieve high performance, it is often necessary to characterize theresponse of the wireless channel. For example, the channel response maybe needed by the transmitter to perform spatial processing (describedbelow) for data transmission to the receiver. The channel response mayalso be needed by the receiver to perform spatial processing on thereceived signals to recover the transmitted data.

In many wireless communication systems, a pilot is transmitted by thetransmitter to assist the receiver in performing a number of functions.The pilot is typically generated based on known symbols and processed ina known manner. The pilot may be used by the receiver for channelestimation, timing and frequency acquisition, data demodulation, and soon.

Various challenges are encountered in the design of a pilot structurefor a MIMO system. As one consideration, the pilot structure needs toaddress the additional dimensionalities created by the multiple transmitand multiple receive antennas. As another consideration, since pilottransmission represents overhead in the MIMO system, it is desirable tominimize pilot transmission to the extent possible. Moreover, if theMIMO system is a multiple-access system that supports communication withmultiple users, then the pilot structure needs to be designed such thatthe pilots needed to support the multiple users do not consume a largeportion of the available system resources.

There is therefore a need in the art for pilots for MIMO systems thataddress the above considerations.

SUMMARY

A method for wireless multiple-input multiple-output (MIMO)communication utilizing orthogonal frequency division multiplexing(OFDM) includes: obtaining a set of pilot symbols for each antenna in aplurality of antennas; obtaining an orthogonal sequence for each antennain the plurality of antennas, wherein the plurality of antennas areassigned different orthogonal sequences; covering the set of pilotsymbols for each antenna with the orthogonal sequence for the antenna toobtain a set of sequences of covered pilot symbols for the antenna,wherein the set of pilot symbols are covered so as to reduce an amountof overhead associated with pilot transmission; processing the set ofsequences of covered pilot symbols for each antenna to obtain a sequenceof OFDM symbols for the antenna; and transmitting the plurality ofsequences of OFDM symbols from the plurality of antennas.

In another aspect, a device for wireless MIMO communication utilizingOFDM includes: a plurality of antennas; a processing circuit configuredto obtain a set of pilot symbols for each antenna in the plurality ofantennas, obtain an orthogonal sequence for each antenna in theplurality of antennas, wherein the plurality of antennas are assigneddifferent orthogonal sequences, cover the set of pilot symbols for eachantenna with the orthogonal sequence for the antenna to obtain a set ofsequences of covered pilot symbols for the antenna, wherein the set ofpilot symbols are covered so as to reduce an amount of overheadassociated with pilot transmission, and process the set of sequences ofcovered pilot symbols for each antenna to obtain a sequence of OFDMsymbols for the antenna; and a transmitter configured to transmit theplurality of sequences of OFDM symbols from the plurality of antennas.

In yet another aspect, a device for wireless MIMO communicationutilizing OFDM includes: means for obtaining a set of pilot symbols foreach antenna in a plurality of antennas; means for obtaining anorthogonal sequence for each antenna in the plurality of antennas,wherein the plurality of antennas are assigned different orthogonalsequences; means for covering the set of pilot symbols for each antennawith the orthogonal sequence for the antenna to obtain a set ofsequences of covered pilot symbols for the antenna, wherein the set ofpilot symbols are covered so as to reduce an amount of overheadassociated with pilot transmission; means for processing the set ofsequences of covered pilot symbols for each antenna to obtain a sequenceof OFDM symbols for the antenna; and means for transmitting theplurality of sequences of OFDM symbols from the plurality of antennas.

In still yet another aspect, a machine-readable storage medium isprovided for use with a wireless MIMO communication system utilizingOFDM. The machine-readable storage medium includes one or moreinstructions which when executed by at least one processing circuitcauses the at least one processing circuit to: obtain a set of pilotsymbols for each antenna in a plurality of antennas; obtain anorthogonal sequence for each antenna in the plurality of antennas,wherein the plurality of antennas are assigned different orthogonalsequences; cover the set of pilot symbols for each antenna with theorthogonal sequence for the antenna to obtain a set of sequences ofcovered pilot symbols for the antenna, wherein the set of pilot symbolsare covered so as to reduce an amount of overhead associated with pilottransmission; process the set of sequences of covered pilot symbols foreach antenna to obtain a sequence of OFDM symbols for the antenna; andtransmit the plurality of sequences of OFDM symbols from the pluralityof antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a multiple-access MIMO system;

FIG. 2 shows an exemplary frame structure for data transmission in a TDDMIMO-OFDM system;

FIG. 3 shows downlink and uplink pilot transmissions for an exemplarypilot transmission scheme;

FIG. 4 shows a block diagram of an access point and a user terminal;

FIG. 5 shows a block diagram of a TX spatial processor that can generatea beacon pilot;

FIG. 6A shows a block diagram of a TX spatial processor that cangenerate a MIMO pilot;

FIG. 6B shows a block diagram of an RX spatial processor that canprovide a channel response estimate based on a received MIMO pilot;

FIG. 7A shows a block diagram of a TX spatial processor that cangenerate a steered reference; and

FIG. 7B shows a block diagram of an RX spatial processor that canprovide a channel response estimate based on a received steeredreference.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a multiple-access MIMO system 100 that supports a number ofusers and is capable of implementing the pilots described herein. MIMOsystem 100 includes a number of access points (APs) 110 that supportcommunication for a number of user terminals (UTs) 120. For simplicity,only two access points 110 a and 110 b are shown in FIG. 1. An accesspoint is generally a fixed station that is used for communicating withthe user terminals. An access point may also be referred to as a basestation or using some other terminology.

User terminals 120 may be dispersed throughout the system. Each userterminal may be a fixed or mobile terminal that can communicate with theaccess point. A user terminal may also be referred to as an accessterminal, a mobile station, a remote station, a user equipment (UE), awireless device, or some other terminology. Each user terminal maycommunicate with one or possibly multiple access points on the downlinkand/or uplink at any given moment. The downlink (i.e., forward link)refers to transmission from the access point to the user terminal, andthe uplink (i.e., reverse link) refers to transmission from the userterminal to the access point. As used herein, an “active” user terminalis one that is receiving a downlink transmission from an access pointand/or transmitting an uplink transmission to the access point.

In FIG. 1, access point 110 a communicates with user terminals 120 athrough 120 f, and access point 110 b communicates with user terminals120 f through 120 k. The assignment of user terminals to access pointsis typically based on received signal strength and not distance. At anygiven moment, a user terminal may receive downlink transmission from oneor multiple access points. A system controller 130 couples to accesspoints 110 and may be designed to perform a number of functions such as(1) coordination and control for the access points coupled to it, (2)routing of data among these access points, and (3) access and control ofcommunication with the user terminals served by these access points.

I. Pilots

Pilots suitable for use in MIMO systems, such as the one shown in FIG.1, are provided herein. These pilots can support various functions thatmay be needed for proper system operation, such as timing and frequencyacquisition, channel estimation, calibration, and so on. The pilots maybe considered as being of different types that are designed and used fordifferent functions. Table 1 lists four types of pilot and their shortdescription for an exemplary pilot design. Fewer, different, and/oradditional pilot types may also be defined, and this is within the scopeof the invention.

TABLE 1 Pilot Types Pilot Type Description Beacon Pilot A pilottransmitted from all transmit antennas and used for timing and frequencyacquisition. MIMO Pilot A pilot transmitted from all transmit antennaswith different orthogonal codes and used for channel estimation. SteeredA pilot transmitted on specific eigenmodes of a MIMO Reference orchannel for a specific user terminal and used for channel Steered Pilotestimation and possibly rate control. Carrier Pilot A pilot used forphase tracking of a carrier signal.

Steered Reference and Steered Pilot are Synonymous Terms.

Various pilot transmission schemes may be devised based on anycombination of these various types of pilot. For example, on thedownlink, an access point may transmit a beacon pilot, a MIMO pilot, anda carrier pilot for all user terminals within its coverage area and mayoptionally transmit a steered reference to any active user terminal thatis receiving a downlink transmission from the access point. On theuplink, a user terminal may transmit a MIMO pilot for calibration andmay transmit a steered reference and a carrier pilot when scheduled(e.g., for downlink and/or uplink data transmissions). The processing totransmit and receive these various types of pilot is described infurther detail below.

The pilots described herein may be used for various types of MIMOsystems. For example, the pilots may be used for (1) single-carrier MIMOsystems, (2) multi-carrier MIMO systems that employ orthogonal frequencydivision multiplexing (OFDM) or some other multi-carrier modulationtechnique, (3) MIMO systems that implement multiple-access techniquessuch as frequency division multiple-access (FDMA), time divisionmultiple-access (TDMA), and code division multiple-access (CDMA), (4)MIMO systems that implement frequency division multiplexing (FDM), timedivision multiplexing (TDM), and/or code division multiplexing (CDM) fordata transmission, (5) MIMO systems that implement time divisionduplexing (TDD), frequency division duplexing (FDD), and/or codedivision duplexing (CDD) for the downlink and uplink channels, and (6)other types of MIMO systems. For clarity, the pilots are described belowfirst for a MIMO system that implements OFDM (i.e., a MIMO-OFDM system)and then for a TDD MIMO-OFDM system.

OFDM effectively partitions the overall system bandwidth into a numberof (N_(F)) orthogonal subbands, which are also referred to as tones,frequency bins, or frequency subchannels. With OFDM, each subband isassociated with a respective subcarrier upon which data may bemodulated. For a MIMO-OFDM system, each subband may be associated with anumber of eigenmodes, and each eigenmode of each subband may be viewedas an independent transmission channel.

For clarity, a specific pilot structure is described below for anexemplary MIMO-OFDM system. In this MIMO-OFDM system, the systembandwidth is partitioned into 64 orthogonal subbands (i.e., N_(F)=64),which are assigned indices of 32 to +31. Of these 64 subbands, 48subbands (e.g., with indices of ±{1, . . . , 6, 8, . . . , 20, 22, . . ., 26}) may be used for data transmission, 4 subbands (e.g., with indicesof ±{7, 21}) may be used for a carrier pilot and possibly signaling, theDC subband (with index of 0) is not used, and the remaining subbands arealso not used and serve as guard subbands. Thus, of the 64 totalsubbands, the 52 “usable” subbands include the 48 data subbands and 4pilot subbands, and the remaining 12 subbands are not used. This OFDMsubband structure is described in further detail in the aforementionedprovisional U.S. Patent Application Ser. No. 60/421,309. Differentnumber of subbands and other OFDM subband structures may also beimplemented for the MIMO-OFDM system, and this is within the scope ofthe invention.

For OFDM, the data to be transmitted on each usable subband is firstmodulated (i.e., symbol mapped) using a particular modulation scheme(e.g., BPSK, QPSK, or M-QAM) selected for use for that subband. Onemodulation symbol may be transmitted on each usable subband in eachsymbol period. Each modulation symbol is a complex value for a specificpoint in a signal constellation corresponding to the selected modulationscheme. Signal values of zero may be sent on the unused subbands. Foreach OFDM symbol period, the modulation symbols for the usable subbandsand zero signal values for the unused subbands (i.e., the modulationsymbols and zeros for all N_(F) subbands) are transformed to the timedomain using an inverse fast Fourier transform (IFFT) to obtain atransformed symbol that comprises N_(F) time-domain samples. To combatinter-symbol interference (ISI), a portion of each transformed symbol isoften repeated (which is also referred to as adding a cyclic prefix) toform a corresponding OFDM symbol, which is then transmitted over thewireless channel. An OFDM symbol period, which is also referred toherein as a symbol period, corresponds to the duration of one OFDMsymbol.

1. Beacon Pilot

The beacon pilot includes a specific set of pilot symbols that istransmitted from each of the N_(T) transmit antennas. The same set ofpilot symbols is transmitted for N_(B) symbol periods designated forbeacon pilot transmission. In general, N_(B) may be any integer value ofone or greater.

In an exemplary embodiment, the set of pilot symbols for the beaconpilot is a set of 12 BPSK modulation symbols for 12 specific subbands,which is referred to as a “B” OFDM symbol. The 12 BPSK modulationsymbols for the B OFDM symbol are given in Table 2. Signal values ofzeros are transmitted on the remaining 52 unused subbands.

TABLE 2 Pilot Symbols Sub- Beacon MIMO band Pilot Pilot Index b(k) p(k). 0 0 . . −26 0 −1 − j −25 0 −1 + j −24   1 + j −1 + j −23 0 −1 + j −220   1 − j −21 0   1 − j −20 −1 − j   1 + j −19 0 −1 − j −18 0 −1 + j −170   1 + j −16   1 + j −1 + j −15 0   1 − j −14 0   1 + j −13 0   1 − j−12 −1 − j   1 − j −11 0 −1 − j −10 0 −1 − j −9 0   1 − j −8 −1 − j −1 −j −7 0   1 + j −6 0 −1 + j −5 0 −1 − j −4   1 + j −1 + j −3 0 −1 + j −20   1 − j −1 0 −1 + j 0 0 0 1 0   1 − j 2 0 −1 − j 3 0 −1 − j 4 −1 − j−1 − j 5 0 −1 + j 6 0   1 + j 7 0 −1 − j 8 −1 − j −1 + j 9 0 −1 − j 10 0−1 − j 11 0   1 + j 12   1 + j   1 − j 13 0 −1 + j 14 0 −1 − j 15 0  1 + j 16   1 + j −1 + j 17 0 −1 + j 18 0   1 − j 19 0   1 + j 20   1 +j −1 + j 21 0   1 + j 22 0 −1 + j 23 0   1 + j 24 1 + j −1 + j 25 0   1− j 26 0 −1 − j . 0 0 . .

For the exemplary embodiment and as shown in Table 2, for the beaconpilot, the BPSK modulation symbol (1+j) is transmitted in subbands −24,−16, −4, 12, 16, 20, and 24, and the BPSK modulation symbol (1+j) istransmitted in subbands −20, −12, −8, 4, and 8. Zero signal values aretransmitted on the remaining 52 subbands for the beacon pilot.

The B OFDM symbol is designed to facilitate system timing and frequencyacquisition by the user terminals. For the exemplary embodiment of the BOFDM symbol described above, only 12 of the 64 total subbands are used,and these subbands are spaced apart by four subbands. This 4-subbandspacing allows the user terminal to have an initial frequency error ofup to two subbands. The beacon pilot allows the user terminal to correctfor its initial coarse frequency error, and to correct its frequency sothat the phase drift over the duration of the beacon pilot is small(e.g., less than 45 degrees over the beacon pilot duration at a samplerate of 20 MHz). If the beacon pilot duration is 8 μsec, then the 45degrees (or less) of phase drift over 8 p sec is equal to 360 degreesover 64 μsec, which is approximately 16 kHz.

The 16 kHz frequency error is typically too large for operation.Additional frequency correction may be obtained using the MIMO pilot andthe carrier pilot. These pilots span a long enough time duration thatthe user terminal frequency can be corrected to within the desiredtarget (e.g., 250 Hz). For example, if a TDD frame is 2 msec (asdescribed below) and if the user terminal frequency is accurate towithin 250 Hz, then there will be less than half a cycle of phase changeover one TDD frame. The phase difference from TDD frame to TDD frame ofthe beacon pilot may be used to lock the frequency of the user terminalto the clock at the access point, thereby effectively reducing thefrequency error to zero.

In general, the set of pilot symbols used for the beacon pilot may bederived using any modulation scheme. Thus, other OFDM symbols derivedusing BPSK or some other modulation scheme may also be used for thebeacon pilot, and this is within the scope of the invention.

In an exemplary design, four transmit antennas are available for beaconpilot transmission. Table 4 lists the OFDM symbols to be transmittedfrom each of the four transmit antennas for a beacon pilot transmissionthat spans two symbol periods.

TABLE 3 Beacon Pilot Symbol Period Antenna 1 Antenna 2 Antenna 3 Antenna4 1 B B B B 2 B B B B

2. MIMO Pilot

The MIMO pilot includes a specific set of pilot symbols that istransmitted from each of the N_(T) transmit antennas. For each transmitantenna, the same set of pilot symbols is transmitted for N_(P) symbolperiods designated for MIMO pilot transmission. However, the set ofpilot symbols for each transmit antenna is “covered” with a uniqueorthogonal sequence or code assigned to that antenna. Covering is aprocess whereby a given pilot or data symbol (or a set of L pilot/datasymbols with the same value) to be transmitted is multiplied by all Lchips of an L-chip orthogonal sequence to obtain L covered symbols,which are then transmitted. Decovering is a complementary processwhereby received symbols are multiplied by the L chips of the sameL-chip orthogonal sequence to obtain L decovered symbols, which are thenaccumulated to obtain an estimate of the transmitted pilot or datasymbol. The covering achieves orthogonality among the N_(T) pilottransmissions from the N_(T) transmit antennas and allows a receiver todistinguish the individual transmit antennas, as described below. Theduration of the MIMO pilot transmission may be dependent on its use, asdescribed below. In general, N_(P) may be any integer value of one orgreater.

One set or different sets of pilot symbols may be used for the N_(T)transmit antennas. In an exemplary embodiment, one set of pilot symbolsis used for all N_(T) transmit antennas for the MIMO pilot and this setincludes 52 QPSK modulation symbols for the 52 usable subbands, which isreferred to as a “P” OFDM symbol. The 52 QPSK modulation symbols for theP OFDM symbol are given in Table 2. Signal values of zero aretransmitted on the remaining 12 unused subbands.

The 52 QPSK modulation symbols form a unique “word” that is designed tofacilitate channel estimation by the user terminals. This unique word isselected to have a minimum peak-to-average variation in a waveformgenerated based on these 52 modulation symbols.

It is well known that OFDM is generally associated with higherpeak-to-average variation in the transmitted waveform than for someother modulation technique (e.g., CDMA). As a result, to avoid clippingof circuitry (e.g., power amplifier) in the transmit chain, OFDM symbolsare typically transmitted at a reduced power level, i.e., backed offfrom the peak transmit power level. The back-off is used to account forvariations in the waveform for these OFDM symbols. By minimizing thepeak-to-average variation in the waveform for the P OFDM symbol, theMIMO pilot may be transmitted at a higher power level (i.e., a smallerback-off may be applied for the MIMO pilot). The higher transmit powerfor the MIMO pilot would then result in improved received signal qualityfor the MIMO pilot at the receiver. The smaller peak-to-averagevariation may also reduce the amount of distortion and non-linearitygenerated by the circuitry in the transmit and receive chains. Thesevarious factors may result in improved accuracy for a channel estimateobtained based on the MIMO pilot.

An OFDM symbol with minimum peak-to-average variation may be obtained invarious manners. For example, a random search may be performed in whicha large number of sets of pilot symbols are randomly formed andevaluated to find the set that has the minimum peak-to-averagevariation. The P OFDM symbol shown in Table 2 represents an exemplaryOFDM symbol that may be used for the MIMO pilot. In general, the set ofpilot symbols used for the MIMO pilot may be derived using anymodulation scheme. Thus, various other OFDM symbols derived using QPSKor some other modulation scheme may also be used for the MIMO pilot, andthis is within the scope of the invention.

Various orthogonal codes may be used to cover the P OFDM symbols sent onthe N_(T) transmit antennas. Examples of such orthogonal codes includeWalsh codes and orthogonal variable spreading factor (OVSF) codes.Pseudo-orthogonal codes and quasi-orthogonal codes may also be used tocover the P OFDM symbols. An example of a pseudo-orthogonal code is theM-sequence that is well known in the art. An example of aquasi-orthogonal code is the quasi-orthogonal function (QOF) defined byIS-2000. In general, various types of codes may be used for covering,some of which are noted above. For simplicity, the term “orthogonalcode” is used herein to generically refer to any type of code suitablefor use for covering pilot symbols. The length (L) of the orthogonalcode is selected to be greater than or equal to the number of transmitantennas (e.g., L≧N_(T)), and L orthogonal codes are available for use.Each transmit antenna is assigned a unique orthogonal code. The N_(P) POFDM symbols to be sent in N_(P) symbol periods from each transmitantenna are covered with the orthogonal code assigned to that transmitantenna.

In an exemplary embodiment, four transmit antennas are available and areassigned 4-chip Walsh sequences of W₁=1111, W₂=1010, W₃=1100, andW₄=1001 for the MIMO pilot. For a given Walsh sequence, a value of “1”indicates that a P OFDM symbol is transmitted and a value of “0”indicates that a −P OFDM symbol is transmitted. For a −P OFDM symbol,each of the 52 QPSK modulation symbols in the P OFDM symbol is inverted(i.e., multiplied with −1). The result of the covering for each transmitantenna is a sequence of covered P OFDM symbols for that transmitantenna. The covering is in effect performed separately for each of thesubbands to generate a sequence of covered pilot symbols for thatsubband. The sequences of covered pilot symbols for all subbands formthe sequence of covered P OFDM symbols.

Table 4 lists the OFDM symbols to be transmitted from each of the fourtransmit antennas for a MIMO pilot transmission that spans four symbolperiods.

TABLE 4 MIMO Pilot Symbol Period Antenna 1 Antenna 2 Antenna 3 Antenna 41 +P +P +P +P 2 +P −P +P −P 3 +P +P −P −P 4 +P −P −P +PFor this set of 4-chip Walsh sequences, the MIMO pilot transmission canoccur in an integer multiple of four symbol periods to ensureorthogonality among the four pilot transmissions from the four transmitantennas. The Walsh sequence is simply repeated for a MIMO pilottransmission that is longer than the length of the Walsh sequence.

The wireless channel for the MIMO-OFDM system may be characterized by aset of channel response matrices H(k), for subband index kεK, whereK=±{1 . . . 26} for the exemplary subband structure described above. Thematrix H(k) for each subband includes N_(T)N_(R) values, {h_(i,j)(k)},for iε{1 . . . N_(R)} and jε{1 . . . N_(T)}, where h_(i,j)(k) representsthe channel gain between the j-th transmit antenna and the i-th receiveantenna.

The MIMO pilot may be used by the receiver to estimate the response ofthe wireless channel. In particular, to recover the pilot sent fromtransmit antenna j and received by receive antenna i, the received OFDMsymbols on antenna i are first multiplied with the Walsh sequenceassigned to transmit antenna j. The “decovered” OFDM symbols for allN_(P) symbol periods for the MIMO pilot are then accumulated, where theaccumulation may be performed individually for each of the 52 usablesubbands. The accumulation may also be performed in the time domain onthe received OFDM symbols (after removing the cyclic prefix in each OFDMsymbol). The accumulation is performed on a sample-by-sample basis overmultiple received OFDM symbols, where the samples for each OFDM symbolcorrespond to different subbands if the accumulation is performed afterthe FFT and to different time indices if the accumulation is performedprior to the FFT. The result of the accumulation is {ĥ_(i,j)(k)}, forkεK, which are estimates of the channel response from transmit antenna jto receive antenna i for the 52 usable subbands. The same processing maybe performed to estimate the channel response from each transmit antennato each receive antenna. The pilot processing provides N_(T)N_(R),complex values for each subband, where the complex values are elementsof the matrix {circumflex over (H)}(k) for the channel response estimatefor that subband.

The pilot processing described above may be performed by the accesspoint to obtain the channel response estimate {circumflex over(H)}_(up)(k) for the uplink, and may also be performed by the userterminal to obtain the channel response estimate {circumflex over(H)}_(dn)(k) for the downlink.

3. Steered Reference or Steered Pilot

For a MIMO-OFDM system, the channel response matrix H(k) for eachsubband may be “diagonalized” to obtain the N_(S) eigenmodes for thatsubband, where N_(S)≦min{N_(T), N_(R)}. This may be achieved byperforming either singular value decomposition on the channel responsematrix H(k) or eigenvalue decomposition on the correlation matrix ofH(k), which is R(k)=H ^(H) (k)H(k). For clarity, singular valuedecomposition is used for the following description.

The singular value decomposition of the channel response matrix II(k)may be expressed as:

H (k)= U (k)Σ(k) V ^(H)(k), for kεK,  Eq (1)

where

-   -   U(k) is an (N_(R)×N_(R)) unitary matrix of left eigenvectors of        H(k);    -   Σ(k) is an (N_(R)×N_(T)) diagonal matrix of singular values of        H(k);    -   V(k) is an (N_(T)×N_(T)) unitary matrix of right eigenvectors of        H(k); and    -   “^(H)” denotes the conjugate transpose.        A unitary matrix M is characterized by the property M ^(H) M=I,        where I is the identity matrix.

Singular value decomposition is described in further detail by GilbertStrang in a book entitled “Linear Algebra and Its Applications,” SecondEdition, Academic Press, 1980. An eigenmode normally refers to atheoretical construct. The MIMO channel may also be viewed as includingN_(S) spatial channels that may be used for data/pilot transmission.Each spatial channel may or may not correspond to an eigenmode,depending on whether or not the spatial processing at the transmitterwas successful in diagonalizing the MIMO channel. For example, datastreams are transmitted on spatial channels (and not eigenmodes) of aMIMO channel if the transmitter has no knowledge or an imperfectestimate of the MIMO channel. For simplicity, the term “eigenmode” isalso used herein to denote the case where an attempt is made todiagonalize the MIMO channel, even though it may not be fully successfuldue to, for example, an imperfect channel estimate.

The diagonal matrix Σ(k) for each subband contains non-negative realvalues along the diagonal and zeros everywhere else. These diagonalentries are referred to as the singular values of H(k) and represent thegains for the independent channels (or eigenmodes) of the MIMO channelfor the k-th subband.

The eigenvalue decomposition may be performed independently for thechannel response matrix H(k) for each of the 52 usable subbands todetermine the N_(S) eigenmodes for the subband. The singular values foreach diagonal matrix Σ(k) may be ordered such that {σ₁(k)≧σ₂(k)≧ . . .≧σ_(N) _(S) (k)}, where σ₁(k) is the largest singular value, σ₂(k) isthe second largest singular value, and so on, and σ_(N) _(S) (k) is thesmallest singular value for the k-th subband. When the singular valuesfor each diagonal matrix Σ(k) are ordered, the eigenvectors (or columns)of the associated matrices U(k) and V(k) are also orderedcorrespondingly. After the ordering, σ₁(k) represents the singular valuefor the best eigenmode for subband k, which is also often referred to asthe “principal” eigenmode.

A “wideband” eigenmode may be defined as the set of same-ordereigenmodes of all subbands after the ordering. Thus, the m-th widebandeigenmode includes the m-th eigenmode of all subbands. Each widebandeigenmode is associated with a respective set of eigenvectors for all ofthe subbands. The “principal” wideband eigenmode is the one associatedwith the largest singular value in each matrix {circumflex over (Σ)}(k)of each subband after the ordering.

The matrix V(k) includes N_(T) eigenvectors that may be used for spatialprocessing at the transmitter, where V(k)=[v ₁(k) v ₂(k) . . . v _(N)_(T) (k)] and v _(m)(k) is the m-th column of V(k), which is theeigenvector for the m-th eigenmode. For a unitary matrix, theeigenvectors are orthogonal to one another. The eigenvectors are alsoreferred to as “steering” vectors.

A steered reference (i.e., a steered pilot) comprises one or more setsof pilot symbols that are transmitted from the N_(T) transmit antennas.In an embodiment, one set of pilot symbols is transmitted on one set ofsubbands for one wideband eigenmode in a given symbol period byperforming spatial processing with a set of steering vectors for thatwideband eigenmode. In another embodiment, multiple sets of pilotsymbols are transmitted on multiple disjoint sets of subbands formultiple wideband eigenmodes in a given symbol period by performingspatial processing with multiple sets of steering vectors for thesewideband eigenmodes (using subband multiplexing, which is describedbelow). For clarity, the following description assumes that one set ofpilot symbols is transmitted on one wideband eigenmode in a given symbolperiod (i.e., no subband multiplexing).

In an embodiment, the set of pilot symbols for the steered reference isthe same P OFDM symbol used for the MIMO pilot. However, various otherOFDM symbols may also be used for the steered reference, and this iswithin the scope of the invention.

A steered reference transmitted for the m-th wideband eigenmode (usingbeam-forming, which is described below) may be expressed as:

x _(m)(k)= v _(m)(k)·p(k), for kεK,  Eq (2)

where

-   -   x _(m) (k) is an (N_(T)×1) transmit vector for the m-th        eigenmode of the k-th subband;    -   v _(m)(k) is the steering vector for the m-th eigenmode of the        k-th subband; and    -   p(k) is the pilot symbol for the k-th subband (e.g., as given in        Table 2).        The vector x _(m)(k) includes N_(T) transmit symbols to be sent        from the N_(T) transmit antennas for the k-th subband.

The steered reference may be used by the receiver to estimate a vectorthat may be used for spatial processing of both data reception andtransmission, as described below. The processing for the steeredreference is described in further detail below.

4. Carrier Pilot

The exemplary OFDM subband structure described above includes four pilotsubbands with indices of −21, −7, 7, and 21. In an embodiment, a carrierpilot is transmitted on the four pilot subbands in all symbol periodsthat are not used for some other types of pilot. The carrier pilot maybe used by the receiver to track the changes in the phase of an RFcarrier signal and drifts in the oscillators at both the transmitter andreceiver. This may provide improved data demodulation performance.

In an embodiment, the carrier pilot comprises four pilot sequences,P_(c1)(n), P_(c2)(n), P_(c3)(n), and P_(c4)(n), that are transmitted onthe four pilot subbands. In an embodiment, the four pilot sequences aredefined as follows:

P _(c1)(n)=P _(c2)(n)=P _(c3)(n)=−P _(c4)(n),  Eq (3)

where n is an index for symbol period (or OFDM symbol).

The pilot sequences may be defined based on various data sequences. Inan embodiment, the pilot sequence P_(c1)(n) is generated based on apolynomial G(x)=x⁷+x⁴+x, where the initial state is set to all ones andthe output bits are mapped to signal values as follows: 1

−1 and 0

1. The pilot sequence P_(c1)(n), for n={1, 2, . . . 127}, may then beexpressed as:

P_(c1)(n)={1,1,1,1,−1,−1,−1,1,−1,−1,−1,−1,1,1,−1,1,−1,−1,1,1,−1,1,1,−1,1,1,1,1,1,1,−1,1,1,1,−1,1,1,−1,−1,1,1,1,−1,1,−1,−1,−1,1,−1,1,−1,−1,1,−1,−1,1,1,1,1,1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,−1,−1,−1,1,1,−1,−1,−1,−1,1,−1,−1,1,−1,1,1,1,1,−1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,−1,1,1,−1,1,−1,1,1,1,−1,−1,1,−1,−1,−1,1,1,1,−1,−1,−1,−1,−1,−1,1}.

The values of “1” and “−1” in the pilot sequence P_(c1)(n) may be mappedto pilot symbols using a particular modulation scheme. For example,using BPSK, a “1” may be mapped to 1+j and a “−1” may be mapped to−(1+j). If there are more than 127 OFDM symbols, then the pilot sequencemay be repeated so that P_(c1)(n)=P_(c1)(n mod 127) for n>127.

In one embodiment, the four pilot sequences P_(c1)(n), P_(c2)(n),P_(c3)(n), and P_(c4) (n) are transmitted on four differentsubband/antenna pairings. Table 5 shows an exemplary assignment of thefour pilot sequences to the four pilot subbands and four transmitantennas.

TABLE 5 Carrier Pilot Subband Antenna 1 Antenna 2 Antenna 3 Antenna 4−21 P_(c1)(n) — — — −7 — P_(c2)(n) — — 7 — — P_(c3)(n) — 21 — — —P_(c4)(n)As shown in Table 5, the pilot sequence P_(c1)(n) is transmitted onsubband −21 of antenna 1, the pilot sequence P_(c2)(n) is transmitted onsubband −7 of antenna 2, the pilot sequence P_(c3) (n) is transmitted onsubband 7 of antenna 3, and the pilot sequence P_(c4)(n) is transmittedon subband 21 of antenna 4. Each pilot sequence is thus transmitted on aunique subband and a unique antenna. This carrier pilot transmissionscheme avoids interference that would result if a pilot sequence istransmitted over multiple transmit antennas on a given subband.

In another embodiment, the four pilot sequences are transmitted on theprincipal eigenmode of their assigned subbands. The spatial processingfor the carrier pilot symbols is similar to the spatial processing forthe steered reference, which is described above and shown in equation(2). To transmit the carrier pilot on the principal eigenmode, thesteering vector v ₁(k) is used for the spatial processing. Thus, thepilot sequence P_(c1)(n) is spatially processed with the steering vectorv ₁(−26), the pilot sequence P_(c2)(n) is spatially processed with thesteering vector v ₁(−7), the pilot sequence P_(c3)(n) is spatiallyprocessed with the steering vector v ₁(7), and the pilot sequence P_(c4)(n) is spatially processed with the steering vector v ₁(26).

II. Pilots for Single-Carrier MIMO Systems

The pilots described herein may also be used for single-carrier MIMOsystems that do not employ OFDM. In that case, much of the descriptionabove still applies but without the subband index k. For the beaconpilot, a specific pilot modulation symbol b may be transmitted from eachof the N_(T) transmit antennas. For the MIMO pilot, a specific pilotmodulation symbol p may be covered with N_(T) orthogonal sequences andtransmitted from the N_(T) transmit antennas. The pilot symbol b may bethe same or different from the pilot symbol p. The steered reference maybe transmitted as shown in equation (2). However, the transmit vectorx_(m), steering vector v _(m), and pilot symbol p are not functions ofsubband index k. The carrier pilot may be transmitted in a time divisionmultiplexed manner or may simply be omitted.

For a MIMO-OFDM system, the cyclic prefix is typically used to ensureorthogonality across the subbands in the presence of delay spread in thesystem, and the orthogonal codes allow for identification of theindividual transmit antennas. For a single-carrier MIMO system, theorthogonal codes are relied upon for both orthogonality and antennaidentification. Thus, the orthogonal codes used for covering the pilotsymbols in a single-carrier MIMO system may be selected to have goodcross-correlation and peak-to-sidelobe properties (i.e., the correlationbetween any two orthogonal sequences used for covering is small in thepresence of delay spread in the system). An example of such anorthogonal code with good cross-correlation and peak-to-sidelobeproperties is the M-sequence and its time-shifted versions. However,other types of codes may also be used for covering pilot symbols for thesingle-carrier MIMO system.

For a wideband single-carrier MIMO system, the steered reference may betransmitted in various manners to account for frequency selective fading(i.e., a frequency response that is not flat across the operating band).Several schemes for transmitting a steered reference in a widebandsingle-carrier MIMO system are described below. In general, atransmitter can transmit a reference waveform that is processed in thesame or similar manner as the processing used to transmit traffic dataon specific wideband eigenmodes. The receiver can then in some mannercorrelate the received waveform against a locally generated copy of thetransmitted reference waveform, and extract information about thechannel that allows the receiver to estimate a channel matched filter.

In a first scheme, a transmitter initially obtains a steering vector v_(m)(k) for an eigenmode. The steering vector v _(m)(k) may be obtainedby periodically transmitting OFDM pilot symbols, by performingfrequency-domain analysis on a received MIMO pilot that has beentransmitted without OFDM, or by some other means. For each value of k,where 1≦k≦N_(F), v _(m)(k) is an N_(T)-vector with N_(T) entries forN_(T) transmit antennas. The transmitter then performs an inverse fastFourier transform on each of the N_(T) vector positions of the steeringvector v _(in)(k), with k as the frequency variable in the IFFTcomputation, to obtain a corresponding time-domain pulse for anassociated transmit antenna. Each vector position of the vector v_(m)(k) includes N_(F) values for N_(F) frequency subbands, and thecorresponding time-domain pulse is a sequence of N_(F) time-domainvalues. The terminal then appends a cyclic prefix to this time-domainpulse to obtain a steered reference pulse for the transmit antenna. Oneset of N_(T) steered reference pulses is generated for each eigenmodeand may be transmitted in the same time interval from all N_(T) transmitantennas. Multiple sets of pulses may be generated for multipleeigenmodes and may be transmitted in a TDM manner.

For the first scheme, a receiver samples the received signal to obtain areceived vector r _(in)(n), removes the cyclic prefix, and performs afast Fourier transform on each vector position of the received vector r_(m)(n) to obtain an estimate of a corresponding entry of H(k)v _(m)(k).Each vector position of the received vector r _(m)(n) (after the cyclicprefix removal) includes N_(F) time-domain samples. The receiver thenuses the estimate of H(k)v _(m)(k) to synthesize a time-domain matchedfilter that may be used to filter a received data transmission. Thetime-domain matched filter includes a matched filter pulse for each ofthe received antennas. The synthesis of the time-domain matched filteris described in commonly assigned U.S. patent application Ser. No.10/017,308, entitled “Time-Domain Transmit and Receive Processing withChannel Eigen-mode Decomposition for MIMO Systems,” filed Dec. 7, 2001.

For the first scheme, the transmitter processing for the steeredreference in a single-carrier MIMO system is similar to the transmitterprocessing for the steered reference in a MIMO-OFDM system. However,other transmission after the steered reference is transmitted on asingle-carrier waveform, such as the one described in the aforementionedU.S. patent application Ser. No. 10/017,308. Moreover, the receiver usesthe steered reference to synthesize time domain matched filters, asdescribed above.

In a second scheme, a transmitter isolates a single multipath componentfor the wideband channel. This may be achieved, for example, bysearching a received MIMO pilot with a sliding correlator in similarmanner as often performed in CDMA systems to search for multipathcomponents. The transmitter then treats this multipath component as anarrowband channel and obtains a single steering vector v _(m) for themultipath component for each eigenmode. Again, multiple steering vectorsmay be generated for multiple eigenmodes for this multipath component.

III. Pilot Structure for a TDD MIMO-OFDM System

The pilots described herein may be used for various MIMO and MIMO-OFDMsystems. These pilots may be used for systems that use a common orseparate frequency bands for the downlink and uplink. For clarity, anexemplary pilot structure for an exemplary MIMO-OFDM system is describedbelow. For this MIMO-OFDM system, the downlink and uplink aretime-division duplexed (TDD) on a single frequency band.

FIG. 2 shows an embodiment of a frame structure 200 that may be used fora TDD MIMO-OFDM system. Data transmission occurs in units of TDD frames,each of which spans a particular time duration (e.g., 2 msec). Each TDDframe is partitioned into a downlink phase and an uplink phase. Thedownlink phase is further partitioned into multiple segments formultiple downlink transport channels. In the embodiment shown in FIG. 2,the downlink transport channels include a broadcast channel (BCH), aforward control channel (FCCH), and a forward channel (FCH). Similarly,the uplink phase is partitioned into multiple segments for multipleuplink transport channels. In the embodiment shown in FIG. 2, the uplinktransport channels include a reverse channel (RCH) and a random accesschannel (RACH).

On the downlink, a BCH segment 210 is used to transmit one BCH protocoldata unit (PDU) 212, which includes a portion 214 for a beacon pilot, aportion 216 for a MIMO pilot, and a portion 218 for a BCH message. TheBCH message carries system parameters for the user terminals in thesystem. An FCCH segment 220 is used to transmit one FCCH PDU, whichcarries assignments for downlink and uplink resources and othersignaling for the user terminals. An FCH segment 230 is used to transmitone or more FCH PDUs 232. Different types of FCH PDU may be defined. Forexample, an FCH PDU 232 a includes a portion 234 a for a pilot and aportion 236 a for a data packet. An FCH PDU 232 b includes a singleportion 236 b for a data packet. An FCH PDU 232 c includes a singleportion 234 c for a pilot.

On the uplink, an RCH segment 240 is used to transmit one or more RCHPDUs 242 on the uplink. Different types of RCH PDU may also be defined.For example, an RCH PDU 242 a includes a single portion 246 a for a datapacket. An RCH PDU 242 b includes a portion 244 b for a pilot and aportion 246 b for a data packet. An RCH PDU 242 c includes a singleportion 244 c for a pilot. An RACH segment 250 is used by the userterminals to gain access to the system and to send short messages on theuplink. An RACH PDU 252 may be sent within RACH segment 250 and includesa portion 254 for a pilot and a portion 256 for a message.

For the embodiment shown in FIG. 2, the beacon and MIMO pilots are senton the downlink in each TDD frame in the BCH segment. A pilot may or maynot be sent in any given FCH/RCH PDU. If the pilot is sent, then it mayspan all or only a portion of the PDU, as shown in FIG. 2. A pilot issent in an RACH PDU to allow the access point to estimate pertinentvectors during access. The pilot portion is also referred to as a“preamble”. The pilot that is sent in any given FCH/RCH PDU may be asteered reference or a MIMO pilot, depending on the purpose for whichthe pilot is used. The pilot sent in an RACH PDU is typically a steeredreference, although a MIMO pilot may also be sent instead. The carrierpilot is transmitted on the pilot subbands and in the portions that arenot used for other pilot transmissions. The carrier pilot is not shownin FIG. 2 for simplicity. The durations of the various portions in FIG.2 are not drawn to scale.

The frame structure and transport channels shown in FIG. 2 are describedin detail in the aforementioned provisional U.S. Patent Application Ser.No. 60/421,309.

1. Calibration

For a TDD MIMO-OFDM system with a shared frequency band, the downlinkand uplink channel responses may be assumed to be reciprocal of oneanother. That is, if H(k) represents a channel response matrix fromantenna array A to antenna array B for subband k, then a reciprocalchannel implies that the coupling from array B to array A is given by H^(T) (k), where H ^(T) denotes the transpose of H. For the TDD MIMO-OFDMsystem, the reciprocal channel characteristics can be exploited tosimplify the channel estimation and spatial processing at both thetransmitter and receiver.

However, the frequency responses of the transmit and receive chains atthe access point are typically different from the frequency responses ofthe transmit and receive chains at the user terminal. An “effective”downlink channel response, H _(dn)(k), and an “effective” uplink channelresponse, H _(up)(k), which include the responses of the applicabletransmit and receive chains, may be expressed as:

H _(dn)(k)= R _(ut)(k) H (k) T _(ap)(k), for kεK, and

H _(up)(k)= R _(ap)(k) H ^(T)(k) T _(ut)(k), for kεK,  Eq (4)

where

-   -   T _(ap)(k) and R _(ap)(k) are N_(ap)×N_(ap) diagonal matrices        for the frequency responses of the transmit chain and receive        chain, respectively, at the access point for subband k;    -   T _(ut)(k) and R _(ut)(k) are N_(ut)×N_(ut) diagonal matrices        for the frequency responses of the transmit chain and receive        chain, respectively, at the user terminal for subband k;    -   N_(ap) is the number of antennas at the access point; and    -   N_(ut) is the number of antennas at the user terminal.

Combining the equations in equation set (4), the following is obtained:

H _(up)(k) K _(ut)(k)=( H _(dn)(k) K _(ap)(k))^(T), for kεK,  Eq (5)

where K _(ut)(k)=T _(ut) ⁻¹(k)R _(ut)(k) and K _(ap)(k)=T_(ap) ⁻¹_(p)(k)R _(ap)(k). Because T _(ut)(k), R _(ut)(k), T _(ap)(k), and R_(ap)(k) are diagonal matrices, {circumflex over (K)}_(ap)(k) and{circumflex over (K)}_(ut)(k) are also diagonal matrices.

Calibration may be performed to obtain estimates, {circumflex over(K)}_(ap)(k) and K _(ut) (k), of the actual diagonal matrices, K_(ap)(k) and K _(ut)(k), for kεK. The matrices {circumflex over(K)}_(ap)(k) and {circumflex over (K)}_(ut)(k) contain correctionfactors that can account for differences in the frequency responses ofthe transmit/receive chains at the access point and user terminal A“calibrated” downlink channel response, H _(cdn)(k), observed by theuser terminal and a “calibrated” uplink channel response, H _(cup)(k),observed by the access point may then be expressed as:

H _(cdn)(k)= H _(dn)(k){circumflex over ( K )}_(ap)(k), for kεK, and  Eq(6a)

H _(cup)(k)= H _(up)(k){circumflex over ( K )}_(ut)(k), for kεK,where  Eq (6b)

H _(cdn)(k)≈ H _(cup) ^(T)(k), for kεK.  Eq (6c)

The accuracy of the relationship in equation (6c) is dependent on theaccuracy of the correction matrices, {circumflex over (K)}_(ap)(k) and{circumflex over (K)}_(ut)(k), which is in turn dependent on the qualityof the estimates of the effective downlink and uplink channel responses,{circumflex over (H)}_(dn)(k) and {circumflex over (H)}_(up)(k), used toderive these correction matrices. A correction vector {circumflex over(k)}_(ut)(k) may be defined to include only the N_(ut) diagonal elementsof {circumflex over (K)}_(ut)(k), and a correction vector {circumflexover (k)}_(ap)(k) may be defined to include only the N_(ap) diagonalelements of {circumflex over (K)}_(ap)(k). Calibration is described indetail in the aforementioned provisional U.S. Patent Application Ser.No. 60/421,462.

The pilots described herein may also be used for MIMO and MIMO-OFDMsystems that do not perform calibration. For clarity, the followingdescription assumes that calibration is performed and that thecorrection matrices {circumflex over (K)}_(ap)(k) and {circumflex over({circumflex over (K)}_(ut)(k) are used in the transmit paths at theaccess point and the user terminal, respectively.

2. Beacon and MIMO Pilots

As shown in FIG. 2, the beacon pilot and MIMO pilot are transmitted onthe downlink in the BCH for each TDD frame. The beacon pilot may be usedby the user terminals for timing and frequency acquisition. The MIMOpilot may be used by the user terminals to (1) obtain an estimate of thedownlink MIMO channel, (2) derive steering vectors for uplinktransmission, and (3) derive a matched filter for downlink transmission,as described below.

In an exemplary pilot transmission scheme, the beacon pilot istransmitted for two symbol periods and the MIMO pilot is transmitted foreight symbol periods at the start of the BCH segment. Table 6 shows thebeacon and MIMO pilots for this exemplary scheme.

TABLE 6 Beacon and MIMO Pilots for BCH Pilot Symbol Type Period Antenna1 Antenna 2 Antenna 3 Antenna 4 Beacon 1 B B B B Pilot 2 B B B B MIMO 3+P +P +P +P Pilot 4 +P −P +P −P 5 +P +P −P −P 6 +P −P −P +P 7 +P +P +P+P 8 +P −P +P −P 9 +P +P −P −P 10 +P −P −P +P

The beacon pilot transmitted on the downlink may be expressed as:

x _(dn,bp)(k)={circumflex over ( k )}_(ap)(k)b(k), for kεK,  Eq (7)

where

-   -   {right arrow over (x)}_(dn,bp)(k) is a transmit vector for        subband k for the beacon pilot; and    -   b(k) is the pilot symbol to be transmitted on subband k for the        beacon pilot, which is given in Table 2.        As shown in equation (7), the beacon pilot is scaled by the        correction vector {circumflex over (k)}_(ap)(k) but not        subjected to any other spatial processing.

The MIMO pilot transmitted on the downlink may be expressed as:

x _(dn,mp,n)(k)={circumflex over ( K )}_(ap)(k) w _(dn,n) p(k), forkεK,  Eq (8)

where

-   -   x _(dn,mp,n)(k) is an (N_(ap)×1) transmit vector for subband k        in symbol period n for the downlink MIMO pilot;    -   w _(dn,n) is an (N_(ap)×1) vector with N_(ap) Walsh chips for        the N_(ap) transmit antennas at the access point in symbol        period n for the downlink MIMO pilot; and    -   p(k) is the pilot symbol to be transmitted on subband k for the        MIMO pilot, which is given in Table 2.

As shown in equation (8), the MIMO pilot is covered by the vector w_(dn,n) and further scaled by the correction matrix {circumflex over(K)}_(ap)(k), but not subjected to any other spatial processing. Thesame Walsh vector w _(dn,n) is used for all subbands, and thus w _(dn,n)is not a function of the subband index k. However, since each Walshsequence is a unique sequence of 4 Walsh chips for 4 symbol periods, w_(dn,n) is a function of symbol period n. The vector w _(dn,n) thusincludes N_(ap) Walsh chips to be used for the N_(ap) transmit antennasat the access point for symbol period n. For the scheme shown in Table6, the four vectors w _(dn,n), for n={3, 4, 5, 6}, for the first foursymbol periods of MIMO pilot transmission on the BCH are w ₃=[1 1 1 1],w ₄=[1 −1 1 −1], w ₅=[1 1 −1 −1], w ₆=[1 −1 −1 1], and the four vectorsw _(dn,n) for n={7, 8, 9, 10}, for the next four symbol periods arerepeated such that w ₇=w ₃, w ₈=w ₄, w ₉=w ₅, and w ₁₀=w ₆.

The MIMO pilot transmitted on the uplink may be expressed as:

x _(up,mp,n)(k)={circumflex over ( K )}_(ut)(k) w _(up,n) p(k), forkεK,  Eq (9)

where x _(up,mp,n)(k) is an (N_(ut)×1) transmit vector for subband k insymbol period n for the uplink MIMO pilot. The Walsh vector w _(up,n)used for the uplink MIMO pilot may be the same or different from theWalsh vector w _(dn,n) used for the downlink MIMO pilot. For example, ifa user terminal is equipped with only two transmit antennas, then w_(up,n) may include two Walsh sequences with length of 2 or greater.

3. Spatial Processing

As described above, the channel response matrix for each subband may bediagonalized to obtain the N_(S) eigenmodes for that subband. Thesingular value decomposition of the calibrated uplink channel responsematrix, H _(cup)(k), may be expressed as:

H _(cup)(k)= U _(ap)(k)Σ(k) V _(ut) ^(H)(k), for kεK,  Eq (10)

where

-   -   U _(ap)(k) is an (N_(ut)×N_(ut)) unitary matrix of left        eigenvectors of H _(cup)(k);    -   Σ(k) is an (N_(ut)×N_(ap)) diagonal matrix of singular values of        H _(cup)(k); and    -   V _(ut)(k) is an (N_(ap)×N_(ap)) unitary matrix of right        eigenvectors of H _(cup) (k).

Similarly, the singular value decomposition of the calibrated downlinkchannel response matrix, H _(cdn)(k), may be expressed as:

H _(cdn)(k)= V* _(ut)(k)Σ(k) U _(ap) ^(T)(k), for kεK,  Eq (11)

where the matrices V*_(ut)(k) and U*_(ap)(k) are unitary matrices ofleft and right eigenvectors, respectively, of H _(cdn)(k).

As shown in equations (10) and (11) and based on the above description,the matrices of left and right eigenvectors for one link are the complexconjugate of the matrices of right and left eigenvectors, respectively,for the other link. For simplicity, reference to the matrices U _(ap)(k)and V _(ut)(k) in the following description may also refer to theirvarious other forms (e.g., V_(ut)(k) may refer to V_(ut)(k), V*_(ut)(k),V _(ut) ^(T)(k), and V _(ut) ^(H), (k)). The matrices U _(ap)(k) and V_(ut)(k) may be used by the access point and user terminal,respectively, for spatial processing and are denoted as such by theirsubscripts.

In an embodiment, the user terminal can estimate the calibrated downlinkchannel response based on a MIMO pilot transmitted by the access point.The user terminal may then perform singular value decomposition of thecalibrated downlink channel response estimate {circumflex over(H)}_(cdn)(k), for kεK, to obtain the diagonal matrix {circumflex over(Σ)}(k) and the matrix {circumflex over (V)}*_(ut)(k) of lefteigenvectors of Ĥ _(cdn)(k) for each subband. This singular valuedecomposition may be given as Ĥ _(cdn)(k)={circumflex over(V)}*_(ut)(k){circumflex over (Σ)}(k){circumflex over (Û)}_(ap) ^(T)(k),where the hat (“̂”) above each matrix indicates that it is an estimate ofthe actual matrix. Similarly, the access point can estimate thecalibrated uplink channel response based on a MIMO pilot transmitted bythe user terminal. The access point may then perform singular valuedecomposition of the calibrated uplink channel response estimate{circumflex over (H)}_(cup)(k), for kεK, to obtain the diagonal matrix{circumflex over (Σ)}(k) and the matrix {circumflex over (U)}_(ap) (k)of left eigenvectors of {circumflex over (H)}_(cup)(k) for each subband.This singular value decomposition may be given as {circumflex over(H)}_(p)(k)={circumflex over (U)}_(ap)(k){circumflex over(Σ)}(k){circumflex over (V)}_(ut) ^(H)(k). The access point and userterminal may also obtain the required eigenvectors based on a steeredreference, as described below.

Data transmission can occur on one or multiple wideband eigenmodes foreach link. The specific number of wideband eigenmodes to use for datatransmission is typically dependent on the channel conditions and may beselected in various manners. For example, the wideband eigenmodes may beselected by using a water-filling procedure that attempts to maximizethe overall throughput by (1) selecting the best set of one or morewideband eigenmodes to use, and (2) distributing the total transmitpower among the selected wideband eigenmode(s).

The MIMO-OFDM system may thus be designed to support multiple operatingmodes, including:

-   -   Spatial multiplexing mode—used to transmit data on multiple        wideband eigenmodes, and    -   Beam-steering mode—used to transmit data on the principal (best)        wideband eigenmode.

Data transmission on multiple wideband eigenmodes may be achieved byperforming spatial processing with multiple sets of eigenvectors in thematrices U _(ap)(k) or V _(ut)(k), for kεK (i.e., one set ofeigenvectors for each wideband eigenmode). Table 7 summarizes thespatial processing at the access point and user terminal for both datatransmission and reception for the spatial multiplexing mode.

TABLE 7 Spatial Processing for Spatial Multiplexing Mode Downlink UplinkAccess Transmit: Receive: Point x _(dn) (k) = 

  (k) 

_(*) (k)s _(dn)

 (k) = 

 (k) 

(k) (k)r _(up) (k) User Receive: Transmit: Terminal

 (k) = 

 (k) 

x _(up) (k) = 

 (k) 

(k)r _(dn) (k) (k)s _(up) (k)In Table 7, s(k) is a “data” vector with up to N_(S) non-zero entriesfor the modulation symbols to be transmitted on the N_(S) eigenmodes ofsubband k, x(k) is a transmit vector for subband k, r(k) is a receivedvector for subband k, and {circumflex over (s)}(k) is an estimate of thetransmitted data vector s(k). The subscripts “dn” and “up” for thesevectors denote downlink and uplink transmissions, respectively.

Data transmission on one wideband eigenmode may be achieved by usingeither “beam-forming” or “beam-steering”. For beam-forming, themodulation symbols are spatially processed with a set of eigenvectors{circumflex over (v)}_(ut,1)(k) or {circumflex over (u)}_(ap,1)(k), forkεK, for the principal wideband eigenmode. For beam-steering, themodulation symbols are spatially processed with a set of “normalized”(or “saturated”) eigenvectors {tilde over (v)}_(ut) (k) or {tilde over(u)}_(ap)(k), for kεK, for the principal wideband eigenmode. Thenormalized eigenvectors {tilde over (v)}_(ut)(k) and {tilde over(u)}_(ap) (k) can be derived as described below.

The spatial processing for the spatial multiplexing and beam-steeringmodes is described in detail in the aforementioned provisional U.S.Patent Application Ser. Nos. 60/421,309 and 60/421,428. The steeredreferences for the spatial multiplexing and beam-steering modes aredescribed below.

4. Steered Reference

For a reciprocal channel (e.g., after calibration has been performed toaccount for differences in the transmit/receive chains at the accesspoint and user terminal), a steered reference may be transmitted by theuser terminal and used by the access point to obtain estimates of both{circumflex over (U)}_(ap) (k) and {circumflex over (Σ)}(k), for kεK,without having to estimate the MIMO channel or perform the singularvalue decomposition. Similarly, a steered reference may be transmittedby the access point and used by the user terminal to obtain estimates ofboth {circumflex over (V)}_(ut)(k) and {circumflex over (Σ)}(k), forkεK.

In an embodiment, the steered reference comprises a set of pilot symbols(e.g., the P OFDM symbol) that is transmitted on one wideband eigenmodein a given symbol period by performing spatial processing with a set ofunnormalized or normalized eigenvectors for that wideband eigenmode. Inan alternative embodiment, the steered reference comprises multiple setsof pilot symbols that are transmitted on multiple wideband eigenmodes inthe same symbol period by performing spatial processing with multiplesets of unnormalized or normalized eigenvectors for these widebandeigenmodes. In any case, the steered reference is transmitted from allN_(ap) antennas at the access point (for the downlink) and all N_(ut)antennas at the user terminal (for the uplink). For clarity, thefollowing description assumes that the steered reference is transmittedfor one wideband eigenmode in a given symbol period.

A. Downlink Steered Reference—Spatial Multiplexing Mode

For the spatial multiplexing mode, the downlink steered referencetransmitted on the m-th wideband eigenmode by the access point may beexpressed as:

x _(dn,sr,m)(k)={circumflex over ( K )}_(ap)(k){circumflex over ( u)}_(ap,m)(k)p(k), for kεK,  Eq (12)

where

-   -   x _(dn,sr,m)(k) is the transmit vector for the k-th subband of        the m-th wideband eigenmode;    -   {circumflex over (u)}_(ap,m)(k) is the eigenvector for the k-th        subband of the m-th wideband eigenmode; and    -   p(k) is the pilot symbol to be transmitted on subband k for the        steered reference (e.g., as given in Table 2).        The steering vector {circumflex over (u)}_(ap,m)(k) is the m-th        column of the matrix {circumflex over (U)}*_(ap)(k), where        {circumflex over (U)}*_(ap)(k)=[{circumflex over (u)}*_(ap,1)(k)        {circumflex over (u)}*_(ap,2)(k) . . . {circumflex over        (u)}*_(ap,N) _(ap) (k)].

The received downlink steered reference at the user terminal for thespatial multiplexing mode may be expressed as:

$\begin{matrix}{{{{\underset{\_}{r}}_{{dn},{sr},m}(k)} = {{{{\underset{\_}{H}}_{dn}(k)}{{\underset{\_}{x}}_{{dn},{sr},m}(k)}} + {{\underset{\_}{n}}_{dn}(k)}}},{{{for}\mspace{14mu} k} \in K},{\approx {{{{\hat{\underset{\_}{v}}}_{{ut},m}^{*}(k)}{\sigma_{m}(k)}{p(k)}} + {{\underset{\_}{n}}_{dn}(k)}}}} & {{Eq}\mspace{14mu} (13)}\end{matrix}$

where σ_(m)(k) is the singular value for the k-th subband of the m-thwideband eigenmode.

B. Downlink Steered Reference—Beam-Steering Mode

For the beam-steering mode, the spatial processing at the transmitter isperformed using a set of “normalized” eigenvectors for the principalwideband eigenmode. The overall transfer function with a normalizedeigenvector {tilde over (u)}_(ap)(k) is different from the overalltransfer function with an unnormalized eigenvector {circumflex over(u)}*_(ap,1)(k) (i.e. H _(dn)(k){circumflex over (K)}_(ap)(k){circumflexover (u)}_(ap,1)(k)≠H _(dn)(k){circumflex over (K)}_(ap)(k){tilde over(u)}_(ap)(k)). A steered reference generated using the set of normalizedeigenvectors for the principal wideband eigenmode may then be sent bythe transmitter and used by the receiver to derive the matched filterfor the beam-steering mode.

For the beam-steering mode, the downlink steered reference transmittedon the principal wideband eigenmode by the access point may be expressedas:

{tilde over ( x )}_(dn,sr)(k)={circumflex over ( K )}_(ap)(k){tilde over( u )}_(ap)(k)p(k), for kεK,  Eq (14)

wherein {tilde over (u)}_(ap)(k) is the normalized eigenvector for thek-th subband of the principal wideband eigenmode, which may be expressedas:

$\begin{matrix}{{{{\underset{\_}{\overset{\sim}{u}}}_{ap}(k)} = \left\lbrack {A\; ^{j\; {\theta_{u\; 1}{(k)}}}\mspace{14mu} A\; ^{j\; {\theta_{u\; 2}{(k)}}}\mspace{14mu} \ldots \mspace{14mu} A\; ^{j\; {\theta_{{uN}_{ap}}{(k)}}}} \right\rbrack^{T}},} & {{Eq}\mspace{14mu} (15)}\end{matrix}$

where A is a constant (e.g., A=1); and

-   -   θ_(ui)(k) is the phase for the k-th subband of the i-th transmit        antenna, which is given as:

$\begin{matrix}{{\theta_{ui}(k)} = {{\angle \; {{\hat{u}}_{{ap},1,i}^{*}(k)}} = {{\tan^{- 1}\left( \frac{{Im}\left\{ {{\hat{u}}_{{ap},1,i}^{*}(k)} \right\}}{{Re}\left\{ {{\hat{u}}_{{ap},1,i}^{*}(k)} \right\}} \right)}.}}} & {{Eq}\mspace{14mu} (16)}\end{matrix}$

As shown in equation (15), the N_(ap) elements of the vector {tilde over(u)}_(ap)(k) have equal magnitudes but possibly different phases. Asshown in equation (16), the phase of each element in the vector {tildeover (u)}_(ap)(k) is obtained from the corresponding element of thevector {circumflex over (u)}*_(ap,1)(k) (i.e., θ_(ui)(k) is obtainedfrom û*_(ap,1,i)(k) where {circumflex over (u)}_(ap,1)(k)=[û_(ap,1,1)(k)û*_(ap,1,2)(k) . . . û*_(ap,1,N) _(ap) (k)]^(T)).

The received downlink steered reference at the user terminal for thebeam-steering mode may be expressed as:

$\begin{matrix}{{{{\overset{\sim}{\underset{\_}{r}}}_{{dn},{sr}}(k)} = {{{{\underset{\_}{H}}_{dn}(k)}{{\overset{\sim}{\underset{\_}{x}}}_{{dn},{sr}}(k)}} + {{\underset{\_}{n}}_{dn}(k)}}},{{{for}\mspace{14mu} k} \in {{K.} \approx {{{{\underset{\_}{H}}_{cdn}(k)}{{\overset{\sim}{\underset{\_}{u}}}_{ap}(k)}{p(k)}} + {{\underset{\_}{n}}_{dn}(k)}}}}} & {{Eq}\mspace{14mu} (17)}\end{matrix}$

C. Uplink Steered Reference—Spatial Multiplexing Mode

For the spatial multiplexing mode, the uplink steered referencetransmitted on the m-th wideband eigenmode by the user terminal may beexpressed as:

x _(up,sr,m)(k)={circumflex over ( K )}_(ut)(k){circumflex over ( v)}_(ut,m)(k)p(k), for kεK.  Eq (18)

The vector {circumflex over (v)}_(ut,m)(k) is the m-th column of thematrix {circumflex over (V)}_(ut)(k), where {circumflex over(V)}(k)=[{circumflex over (v)}_(ut,1)(k) {circumflex over (v)}_(ut,2)(k). . . {circumflex over (v)}_(ut,N) _(al) (k)].

The received uplink steered reference at the access point for thespatial multiplexing mode may be expressed as:

$\begin{matrix}{{{{\underset{\_}{r}}_{{up},{sr},m}(k)} = {{{{\underset{\_}{H}}_{up}(k)}{{\underset{\_}{x}}_{{up},{sr},m}(k)}} + {{\underset{\_}{n}}_{up}(k)}}},{{{for}\mspace{14mu} k} \in {{K.} \approx {{{{\hat{\underset{\_}{u}}}_{{ap},m}(k)}{\sigma_{m}(k)}{p(k)}} + {{\underset{\_}{n}}_{up}(k)}}}}} & {{Eq}\mspace{14mu} (19)}\end{matrix}$

D. Uplink Steered Reference—Beam-Steering Mode

For the beam-steering mode, the uplink steered reference transmitted onthe principal wideband eigenmode by the user terminal may be expressedas:

{tilde over ( x )}_(up,sr)(k)={circumflex over ( K )}_(ut)(k){tilde over( v )}_(ut)(k)p(k), for kεK.  Eq (20)

The normalized eigenvector {tilde over (v)}_(ut) (k) for the k-thsubband for the principal wideband eigenmode may be expressed as:

$\begin{matrix}{{{{\overset{\sim}{\underset{\_}{v}}}_{ut}(k)} = \begin{bmatrix}{A\; ^{{j\theta}_{v\; 1}{(k)}}} & {A\; ^{{j\theta}_{v\; 2}{(k)}}} & \ldots & {A\; ^{{j\theta}_{{vN}_{ut}}{(k)}}}\end{bmatrix}^{T}},{where}} & {{Eq}\mspace{14mu} (21)} \\{{\theta_{vi}(k)} = {{\angle \; {{\hat{v}}_{{ut},1,i}(k)}} = {{\tan^{- 1}\left( \frac{{Im}\left\{ {{\hat{v}}_{{ut},1,i}(k)} \right\}}{{Re}\left\{ {{\hat{v}}_{{ut},1,i}(k)} \right\}} \right)}.}}} & {{Eq}\mspace{14mu} (22)}\end{matrix}$

As shown in equation (22), the phase of each element in the vector{tilde over (v)}_(ut)(k) is obtained from the corresponding element ofthe eigenvector {circumflex over (v)}_(ut,1)(k).

The received uplink steered reference at the access point for thebeam-steering mode may be expressed as:

$\begin{matrix}{{{{\overset{\sim}{\underset{\_}{r}}}_{{up},{sr}}(k)} = {{{\underset{\_}{H}}_{up}\; (k){{\overset{\sim}{\underset{\_}{x}}}_{{up},{sr}}(k)}} + {{\underset{\_}{n}}_{up}(k)}}},{{{for}\mspace{14mu} k} \in {{K.} \approx {{{{\underset{\_}{H}}_{cup}(k)}{{\overset{\sim}{\underset{\_}{v}}}_{ut}(k)}{p(k)}} + {{\underset{\_}{n}}_{up}(k)}}}}} & {{Eq}\mspace{14mu} (23)}\end{matrix}$

Table 8 summarizes the spatial processing at the access point and userterminal for the steered reference for the spatial multiplexing andbeam-steering modes.

TABLE 8 Spatial Processing for Steered Reference Spatial MultiplexingMode Beam-Steering Mode Access x _(dn,sr,m) (k) = 

 (k) 

 (k) = 

 (k) 

Point (k)p(k) (k) p(k) User x _(up,sr,m) (k) = 

 (k) 

 (k) = 

 (k) 

  Terminal (k)p(k) (k)p(k)

E. Steered Reference Transmission

For the exemplary frame structure shown in FIG. 2, the steered referencemay be transmitted in the preamble or pilot portion of an FCH PDU (forthe downlink) or an RCH PDU (for the uplink). The steered reference maybe transmitted in various manners.

In one embodiment, for the spatial multiplexing mode, the steeredreference is transmitted for one or more wideband eigenmodes for eachTDD frame. The specific number of wideband eigenmodes to transmit ineach TDD frame may be dependent on the duration of the steeredreference. Table 9 lists the wideband eigenmodes used for the steeredreference in the preamble of an FCH/RCH PDU for various preamble sizes,for an exemplary design with four transmit antennas.

TABLE 9 Preamble Size Wideband Eigenmodes Used 0 OFDM symbol no preamble1 OFDM symbol wideband eigenmode m, where m = frame counter mod 4 4 OFDMsymbols cycle through all 4 wideband eigenmodes in the preamble 8 OFDMsymbols cycle through all 4 wideband eigenmodes twice in the preamble

As shown in Table 9, the steered reference is transmitted for all fourwideband eigenmodes within the same TDD frame when the preamble size isfour or eight symbol periods. The steered reference transmitted in thepreamble of an FCH PDU by the access point for the n-th symbol periodmay be expressed as:

x _(dn,sr,n)(k)={circumflex over (K)} _(ap)(k){circumflex over(u)}*_([(n−1)mod 4]+1)(k)p(k), for kεK and nε{1 . . . L},  Eq (24)

where L is the preamble size (e.g., L=0, 1, 4, or 8 for the exemplarydesign shown in Table 9).

The steered reference transmitted in the preamble of an RCH PDU by theuser terminal for the n-th symbol period may be expressed as:

x _(up,sr,n)(k)={circumflex over (K)} _(ut)(k){circumflex over (v)}_(ut,[(n−1)mod 4]+1)(k)p(k), for kεK and nε{1 . . . L}.  Eq (25)

In equations (24) and (25), the four wideband eigenmodes are cycledthrough in each 4-symbol period by the “mod” operation for the steeringvector. This scheme may be used if the channel changes more rapidlyand/or during the early part of a communication session when a goodchannel estimate needs to be obtained quickly for proper systemoperation.

In another embodiment, the steered reference is transmitted for onewideband eigenmode for each TDD frame. The steered reference for fourwideband eigenmodes may be cycled through in four TDD frames. Forexample, the steering vectors {circumflex over (v)} _(ut,1)(k),{circumflex over (v)}_(ut,2)(k), {circumflex over (v)}_(ut,3) and{circumflex over (v)} _(ut,4)(k) may be used for four consecutive TDDframes by the user terminal. The particular steering vector to use forthe steered reference in each TDD frame may be specified by a framecounter, which may be sent in the BCH message. This scheme may allow ashorter preamble to be used for the FCH and RCH PDUs. However, a longertime period may be needed to obtain a good estimate of the channel.

For the beam-steering mode, the normalized steering vector for theprincipal wideband eigenmode is used for the steered reference, as shownin equations (14) and (20). The duration of the steered reference may beselected, for example, based on the channel conditions.

While operating in the beam-steering mode, the user terminal maytransmit multiple symbols of steered reference, for example, one or moresymbols using the normalized eigenvector {tilde over (v)}_(ut)(k), oneor more symbols using the eigenvector {circumflex over (v)}_(ut,1)(k)for the principal eigenmode, and possibly one or more symbols using theeigenvectors for the other eigenmodes. The steered reference symbolsgenerated with {tilde over (v)}_(ut)(k) may be used by the access pointto derive an uplink matched filter vector. This vector is used by theaccess point to perform matched filtering of the uplink datatransmission sent by the user terminal using beam-steering. The steeredreference symbols generated with {circumflex over (v)}_(ut,1)(k) may beused to obtain {circumflex over (u)}_(ap,1)(k), which may then be usedto derive the normalized eigenvector {tilde over (u)}_(ap)(k) that isused for beam-steering on the downlink. The steered reference symbolsgenerated with the eigenvectors {circumflex over (v)}_(ut,2)(k) through{circumflex over (v)}_(ut,N) _(S) (k) for the other eigenmodes may beused by the access point to obtain {circumflex over (u)}_(ap,2)(k)through {circumflex over (u)}_(ap,N) _(S) (k) and the singular valueestimates for these other eigenmodes. This information may then be usedby the access point to determine whether to use the spatial multiplexingmode or the beam-steering mode for downlink data transmission.

For the downlink, the user terminal may derive a downlink matched filtervector for the beam-steering mode based on the calibrated downlinkchannel response estimate {circumflex over (H)}_(cdn)(k). In particular,the user terminal has {circumflex over (u)}*_(ap,1)(k) from the singularvalue decomposition of {circumflex over (H)}_(cdn)(k) and can thenderive the normalized eigenvector {tilde over (u)}_(ap)(k). The userterminal can then multiply {tilde over (u)}_(ap) (k) with {circumflexover (H)}_(cdn)(k) to obtain {circumflex over (H)}_(cdn)(k){tilde over(u)}_(ap)(k), and may then derive the downlink matched filter vector forthe beam-steering mode based on {circumflex over (H)}_(cdn)(k){tildeover (u)}_(ap) (k). Alternatively, a steered reference may be sent bythe access point using the normalized eigenvector {tilde over(v)}_(ap)(k), and this steered reference may be processed by the userterminal in the manner described above to obtain the downlink matchedfilter vector for the beam-steering mode.

F. Subband Multiplexing for Steered Reference

For both the spatial multiplexing and beam-steering modes, the steeredreference may also be transmitted for multiple wideband eigenmodes for agiven symbol period using subband multiplexing. The usable subbands maybe partitioned into multiple disjoint sets of subbands, one set for eachwideband eigenmode selected for steered reference transmission. Each setof subbands may then be used to transmit a steered reference for theassociated wideband eigenmode. For simplicity, the term “widebandeigenmode” is used here even though the steered reference is sent ononly a subset of all usable subbands.

For example, the steered reference may be transmitted on all fourwideband eigenmodes in one symbol period. In this case, the 52 usablesubbands may be partitioned into four disjoint sets (e.g., labeled assets 1, 2, 3, and 4), with each set including 13 subbands. The 13subbands in each set may be uniformly distributed across 52 usablesubbands. The steered reference for the principal wideband eigenmode maythen be transmitted on the 13 subbands in set 1, steered reference forthe second wideband eigenmode may be transmitted on the 13 subbands inset 2, steered reference for the third wideband eigenmode may betransmitted on the 13 subbands in set 3, and steered reference for thefourth wideband eigenmode may be transmitted on the 13 subbands in set4.

If the steered reference is sent on only a subset of all usable subbandsfor a given wideband eigenmode, then interpolation or some othertechnique may be used to obtain estimates for the subbands not used forsteered reference transmission for that wideband eigenmode.

In general, the multiple sets of subbands may include the same ordifferent number of subbands. For example, the number of subbands toinclude in each set may be dependent on the SNR of the widebandeigenmode associated with the set (e.g., more subbands may be assignedto a set associated with a poor quality wideband eigenmode). Moreover,the subbands in each set may be uniformly or non-uniformly distributedacross the usable subbands. The multiple sets of subbands may also beassociated with the same or different sets of pilot symbols.

Subband multiplexing may be used to reduce the amount of overhead neededto transmit the steered reference, which can improve the efficiency ofthe system.

G. Channel Estimation with the Steered Reference

As shown in equation (13), at the user terminal, the received downlinksteered reference for the spatial multiplexing mode (in the absence ofnoise) is approximately {circumflex over (v)}*_(ut,m)(k)σ_(m)(k)p(k).Similarly, as shown in equation (19), at the access point, the receiveduplink steered reference for the spatial multiplexing mode (in theabsence of noise) is approximately {circumflex over (u)}(k)σ_(m)(k)p(k).The access point can thus obtain an estimate of {circumflex over(u)}_(ap,m)(k) and σ_(m)(k) based on a steered reference sent by theuser terminal, and vice versa.

Various techniques may be used to process a steered reference. Forclarity, the following description is for the processing of an uplinksteered reference. The received vector at the access point is given inequation (19), which is r _(up,sr,m)(k)≈{circumflex over(u)}_(ap,m)(k)σ_(m)(k)p(k)+n _(up) (k).

In one embodiment, to obtain an estimate of {circumflex over(u)}_(ap,m)(k), the received vector r _(up,sr,m)(k) for the steeredreference sent on the m-th wideband eigenmode is first multiplied withthe complex conjugate of the pilot symbol, p*(k), that is used for thesteered reference. The result may then be integrated over multiplereceived steered reference symbols for each wideband eigenmode to obtainan estimate of {circumflex over (u)}_(ap,m)(k)σ_(m) (k), which is ascaled left eigenvector of {circumflex over (H)}_(cup)(k) for the m-thwideband eigenmode. Each of the N_(ap) entries of the vector {circumflexover (u)}_(ap,m)(k) is obtained based on a corresponding one of theN_(ap) entries for the vector r _(up,m)(k), where the N_(ap) entries ofr _(up,m)(k) are the symbols received from the N_(ap) antennas at theaccess point. Since eigenvectors have unit power, the singular valueσ_(m)(k) may be estimated based on the received power of the steeredreference, which can be measured for each subband of each widebandeigenmode. The singular value estimate {circumflex over (σ)}_(m)(k) isthen equal to the square root of the received power divided by themagnitude of the pilot symbol p(k).

In another embodiment, a minimum mean square error (MMSE) technique isused to obtain an estimate of the vector {circumflex over (u)}_(ap,m)(k)based on the received vector r _(up,sr,m)(k) for the steered reference.Since the pilot symbols p(k) are known, the access point can derive anestimate of û_(ap,m)(k) such that the mean square error between thereceived pilot symbols (obtained after performing the matched filteringon the received vector r _(up,sr,m)(k)) and the transmitted pilotsymbols is minimized. The use of the MMSE technique for spatialprocessing at the receiver is described in commonly assigned U.S. patentapplication Ser. No. 09/993,087, entitled “Multiple-AccessMultiple-Input Multiple-Output (MIMO) Communication System,” filed Nov.6, 2001.

The steered reference is sent for one wideband eigenmode in any givensymbol period (without subband multiplexing), and may in turn be used toobtain an estimate of one eigenvector for each subband of that widebandeigenmode. Thus, the receiver is able to obtain an estimate of only oneeigenvector in a unitary matrix for any given symbol period. Sinceestimates of multiple eigenvectors for the unitary matrix are obtainedover different symbol periods, and due to noise and other sources ofdegradation in the wireless channel, the estimated eigenvectors for theunitary matrix (which are individually derived) are not likely beorthogonal to one another. The estimated eigenvectors may thereafter beused for matched filtering of a data transmission received on the samelink and/or spatial processing of a data transmission sent on the otherlink. In this case, any errors in orthogonality among these estimatedeigenvectors would result in cross-talk among the data streams sent onthe eigenmodes corresponding to the eigenvectors. The cross-talk maydegrade performance.

In an embodiment, the estimated eigenvectors for each unitary matrix areforced to be orthogonal to each other. The orthogonalization of theeigenvectors may be achieved using the Gram-Schmidt technique, which isdescribed in detail in the aforementioned reference from Gilbert Strang,or some other technique.

Other techniques to process the steered reference may also be used, andthis is within the scope of the invention.

The access point can thus estimate both {circumflex over (U)}_(ap)(k)and {circumflex over (Σ)}(k) based on the steered reference sent by theuser terminal, without having to estimate the uplink channel response orperform singular value decomposition of {circumflex over (H)}_(cup)(k).

The processing at the user terminal to estimate the matrices {circumflexover (V)}_(ut)(k) and {circumflex over (Σ)}(k), for kεK, based on thedownlink steered reference may be performed similar to that describedabove for the uplink steered reference.

For the beam-steering mode, on the uplink, the received vector up r_(up,sr,m)(k) for the steered reference may be processed by the accesspoint in a similar manner to obtain an estimate of H _(cup)(k){tildeover (v)}(k). The conjugate transpose of this estimate is then thematched filter for the uplink transmission in the beam-steering mode. Onthe downlink, the received vector {tilde over (r)}_(dn,sr,m)(k) for thesteered reference may be processed by the user terminal in a similarmanner to obtain an estimate of H _(cdn)(k){tilde over (u)}_(ap)(k). Theconjugate transpose of this estimate is then the matched filter for thedownlink transmission in the beam-steering mode.

5. Carrier Pilot

The carrier pilot may be transmitted on the pilot subbands in variousmanners for the TDD frame structure shown in FIG. 2. In one embodiment,the four pilot sequences are reset for each transport channel Thus, onthe downlink, the pilot sequences are reset for the first OFDM symbol ofthe BCH message, reset again for the first OFDM symbol of the FCCHmessage, and reset for the first OFDM symbol sent on the FCH. In anotherembodiment, the pilot sequences are reset at the start of each TDD frameand repeated as often as needed. For this embodiment, the pilotsequences may be stalled during the preamble portions of the BCH andFCH. The carrier pilot may also be transmitted in other manners, andthis is within the scope of the invention.

6. Pilot Transmission Scheme

Four types of pilot have been described above and may be used for MIMOand MIMO-OFDM systems. These four different types of pilot may betransmitted in various manners.

FIG. 3 shows downlink and uplink pilot transmissions for an exemplarypilot transmission scheme. Generally, block 310 corresponds to a systemaccess phase, block 320 corresponds to a calibration phase, and block330 corresponds to a normal operation phase.

A beacon pilot and a MIMO pilot are transmitted on the downlink by theaccess point in each TDD frame (block 312) to allow all user terminalsin the system to acquire the system frequency and timing and to estimatethe downlink channel (block 314). Block 314 may be performed asnecessary to access the system.

Calibration may be performed prior to normal operation to calibrate outdifferences in the transmit/receive chains at the access point and userterminal. For the calibration, MIMO pilots may be transmitted by boththe access point and the user terminal (blocks 322 and 326). The uplinkMIMO pilot may be used by the access point to derive an estimate of theuplink channel (block 324), and the downlink MIMO pilot may be used bythe user terminal to derive or update an estimate of the downlinkchannel (block 328). The downlink and uplink channel estimates are thenused to derive the correction factors for the access point and the userterminal.

During normal operation, a steered reference may be transmitted on theuplink by the user terminal (1) if and when it desires a datatransmission or (2) if it is scheduled for data transmission (block332). The uplink steered reference may be used by the access point toestimate the pertinent unitary and diagonal matrices for the userterminal (block 334). A steered reference may optionally be transmittedby the access point to the user terminal (as shown by dashed block 336).The user terminal can continually update its estimate of the downlinkchannel based on the downlink MIMO pilot and update the pertinentunitary and diagonal matrices based on the downlink steered reference(if transmitted) (block 338). Carrier pilots are transmitted by theaccess point (block 340) and the user terminal (block 344) on the pilotsubbands during portions that are not used for other pilots. Thedownlink carrier pilot is used by the user terminal to track the phaseof the downlink carrier signal (block 342), and the uplink carrier pilotis used by the access point to track the phase of the uplink carriersignal (block 346).

For the pilot transmission scheme shown in FIG. 3, the user terminalestimates the downlink channel response based on the downlink MIMO pilotand transmits a steered reference on the uplink, which is then used bythe access point to estimate the pertinent unitary and diagonal matricesfor the user terminal. In certain instances, the user terminal may haveobtained a bad estimate of the downlink channel response, in which casethe uplink steered reference may be equally bad or possibly worse. Inthe worst case, the steering vector used by the user terminal may resultin a beam null being pointed at the access point. If this occurs, thenthe access point would not be able to detect the uplink steeredreference. To avoid this situation, the user terminal can perturb thephases of the N_(ut) elements of the steering vector it uses for thesteered reference in situations where it detects that the access pointis not receiving the steered reference properly. For example, if theuser terminal is designated to transmit an uplink steered reference aspart of a system access procedure, and if access to the system is notgained after a particular number of access attempts, then the userterminal can start to perturb the phases of the steering vectorelements.

Various other pilot transmission schemes may also be implemented forMIMO and MIMO-OFDM systems, and this is within the scope of theinvention. For example, the beacon and carrier pilots may be combinedinto a single pilot that can be used for frequency and timingacquisition and carrier phase tracking. As another example, the activeuser terminals may transmit MIMO pilots, instead of steered references,on the uplink.

IV. MIMO-OFDM System

FIG. 4 shows a block diagram of an embodiment of an access point 110 xand a user terminal 120 x in MIMO-OFDM system 100. For clarity, in thisembodiment, access point 110 x is equipped with four antennas that canbe used for data transmission and reception, and user terminal 120 x isalso equipped with four antennas for data transmission/reception. Ingeneral, the access point and user terminal may each be equipped withany number of transmit antennas and any number of receive antennas.

On the downlink, at access point 110 x, a transmit (TX) data processor414 receives traffic data from a data source 412 and signaling and otherdata from a controller 430. TX data processor 414 formats, codes,interleaves, and modulates (i.e., symbol maps) the data to providemodulation symbols. A TX spatial processor 420 receives and multiplexesthe modulation symbols from TX data processor 414 with pilot symbols,performs the required spatial processing, and provides four streams oftransmit symbols for the four transmit antennas.

Each modulator (MOD) 422 receives and processes a respective transmitsymbol stream to provide a corresponding downlink modulated signal. Thefour downlink modulated signals from modulators 422 a through 422 d arethen transmitted from antennas 424 a through 424 d, respectively.

At user terminal 120 x, four antennas 452 a through 452 d receive thetransmitted downlink modulated signals, and each antenna provides areceived signal to a respective demodulator (DEMOD) 454. Eachdemodulator 454 performs processing complementary to that performed atmodulator 422 and provides received symbols. A receive (RX) spatialprocessor 460 then performs spatial processing on the received symbolsfrom all demodulators 454 a through 454 d to provide recovered symbols,which are estimates of the modulation symbols transmitted by the accesspoint. An RX data processor 470 further processes (e.g., symbol demaps,deinterleaves, and decodes) the recovered symbols to provide decodeddata, which may be provided to a data sink 472 for storage and/or acontroller 480 for further processing.

The processing for the uplink may be the same or different from theprocessing for the downlink. Data and signaling are processed (e.g.,coded, interleaved, and modulated) by a TX data processor 488,multiplexed with pilot symbols, and further spatially processed by a TXspatial processor 490. The transmit symbols from TX spatial processor490 are further processed by modulators 454 a through 454 d to generatefour uplink modulated signals, which are then transmitted via antennas452 a through 452 d.

At access point 410, the uplink modulated signals are received byantennas 424 a through 424 d, demodulated by demodulators 422 a through422 d, and processed by an RX spatial processor 440 and an RX dataprocessor 442 in a complementary manner to that performed at the userterminal. The decoded data for the uplink may be provided to a data sink444 for storage and/or controller 430 for further processing.

Controllers 430 and 480 control the operation of various processingunits at the access point and user terminal, respectively. Memory units432 and 482 store data and program codes used by controllers 430 and480, respectively.

FIG. 5 shows a block diagram of a TX spatial processor 420 a that cangenerate a beacon pilot and which may be implemented within TX spatialprocessor 420 in FIG. 4. Processor 420 a includes a number of beaconpilot subband processors 510 a through 510 k, one for each subband usedto transmit the beacon pilot. Each subband processor 510 receives apilot symbol b(k) for the beacon pilot and a correction matrix{circumflex over (K)}_(ap)(k) for the associated subband.

Within each subband processor 510, the pilot symbol b(k) is scaled byfour multipliers 514 a through 514 d with four correction factors{circumflex over (K)}_(ap,1)(k) through {circumflex over (K)}_(ap,4)(k),respectively, from the matrix {circumflex over (K)}_(ap)(k). Eachmultiplier 514 performs complex multiplication of the complex pilotsymbol with a respective complex correction factor. The scaled pilotsymbols from multipliers 514 a through 514 d are then provided to fourbuffers/multiplexers 520 a through 520 d, respectively, which alsoreceive the scaled pilot symbols from other subband processors 510. Eachbuffer/multiplexer 520 multiplexes the scaled pilot symbols for allsubbands used for beacon pilot transmission and signal values of zerofor the unused subbands and provides a stream of transmit symbols forthe associated transmit antenna.

FIG. 6A shows a block diagram of a TX spatial processor 420 b that cangenerate a MIMO pilot. Processor 420 b may be implemented within TXspatial processor 420 or 490 in FIG. 4, but for clarity is describedbelow for an implementation in TX spatial processor 420. Processor 420 bincludes a number of MIMO pilot subband processors 610 a through 610 k,one for each subband used to transmit the MIMO pilot. Each subbandprocessor 610 receives a pilot symbol p(k) for the MIMO pilot and acorrection matrix {circumflex over (K)}_(ap)(k) for the associatedsubband. Each subband processor 610 also receives four Walsh sequences,w₁ through w₄, assigned to the four transmit antennas at the accesspoint.

Within each subband processor 610, the complex pilot symbol p(k) iscovered by the four Walsh sequences w₁ through w₄ by four complexmultipliers 612 a through 612 d, respectively. The covered pilot symbolsare further scaled by four complex multipliers 614 a through 614 d withfour complex correction factors {circumflex over (K)}_(ap,1)(k) through{circumflex over (K)}_(ap,4)(k), respectively, from the matrix{circumflex over (K)}_(ap)(k). The scaled pilot symbols from multipliers614 a through 614 d are then provided to four buffers/multiplexers 620 athrough 620 d, respectively. The subsequent processing is as describedabove for FIG. 5.

For an implementation of processor 420 b in TX spatial processor 490,the number of Walsh sequences to use is dependent on the number oftransmit antennas available at the user terminal. Moreover, the scalingis performed with the correction factors from the matrix {circumflexover (K)}_(ut)(k) for the user terminal.

FIG. 6B shows a block diagram of an RX spatial processor 460 b that canprovide a channel response estimate based on a received MIMO pilot.Processor 460 b may be implemented within RX spatial processor 440 or460 in FIG. 4, but for clarity is described below for an implementationin RX spatial processor 460. Processor 460 b includes a number of MIMOpilot subband processors 650 a through 650 k, one for each subband usedfor MIMO pilot transmission. Each MIMO pilot subband processor 650receives a vector r(k) and a conjugated pilot symbol p*(k) for theassociated subband. Each subband processor 650 also receives the fourWalsh sequences w, through w₄ assigned to the four transmit antennas atthe access point.

Each MIMO pilot subband processor 650 includes four MIMO pilotsubband/antenna processors 660 a through 660 d for the four receiveantennas at the user terminal. Each processor 660 receives one entryr_(i)(k) of the vector r(k). Within each processor 660, the receivedsymbol r_(i)(k) is first multiplied with the conjugated pilot symbolp*(k) by a complex multiplier 662. The output of multiplier 662 isfurther multiplied with the four Walsh sequences w₁ through w₄ by fourcomplex multipliers 664 a through 664 d, respectively. The outputs frommultipliers 664 a through 664 d are then accumulated by accumulators 666a through 666 d, respectively, for the duration of the MIMO pilottransmission. Each pair of multiplier 664 and accumulator 666 performsdecovering for one transmit antenna at the access point. The output fromeach accumulator 666 represents an estimate ĥ_(i,j)(k) of the channelgain from transmit antenna j to receive antenna i for subband k. Thechannel response estimates {ĥ_(i,j)(k)}, for i={1, 2, 3, 4} and j={1, 2,3, 4}, may further be averaged over multiple MIMO pilot transmissions(not shown in FIG. 6B) to provide a more accurate estimate of thechannel response.

As shown in FIG. 6B, each MIMO pilot subband/antenna processor 660provides a row vector {circumflex over (h)}_(cdn,i)(k)=[ĵ_(i,1)(k)ĥ_(i,2)(k) ĥ_(i,3)(k) ĥ_(i,4)(k)] for the associated receive antenna i,where {circumflex over (h)}_(cdn,i)(k) is the i-th row of the calibratedchannel response estimate {circumflex over (H)}_(cdn)(k) for thedownlink (assuming that the access point applied its correction matrix{circumflex over (K)}_(ap)(k)). Processors 660 a through 660 dcollectively provide the four rows of the calibrated channel responsematrix {circumflex over (H)}_(cdn)(k).

FIG. 7A shows a block diagram of a TX spatial processor 420 c that cangenerate a steered reference. Processor 420 c may also be implementedwithin TX spatial processor 420 or 490 in FIG. 4, but for clarity isdescribed below for an implementation in TX spatial processor 420.Processor 420 c includes a number of steered reference subbandprocessors 710 a through 710 k, one for each subband used to transmitthe steered reference. To generate the steered reference for the spatialmultiplexing mode, each subband processor 710 receives a pilot symbolp(k), the steering vector {circumflex over (u)}_(ap,m)(k) for eachwideband eigenmode on which the steered reference is to be transmitted,and a correction matrix {circumflex over (K)}_(ap)(k) for the associatedsubband.

Within each subband processor 710, the pilot symbol p(k) is multipliedwith the four elements through û_(ap,1,m)(k) of the steering vectorû*_(ap,4,m)(k) for the m-th wideband eigenmode by four complexmultipliers 712 a through 712 d, respectively. The outputs frommultipliers 712 a through 712 d are further scaled by four complexmultipliers 714 a through 714 d with four correction factors {circumflexover (K)}_(ap,1)(k) through {circumflex over (K)}_(ap,4)(k),respectively, from the matrix {circumflex over (K)}_(ap)(k). The scaledpilot symbols from multipliers 714 a through 714 d are then provided tofour buffers/multiplexers 720 a through 720 d, respectively. Thesubsequent processing is as described above.

To generate the steered reference on the downlink for the beam-steeringmode, each subband processor 710 would receive a normalized steeringvector {tilde over (u)}_(ap)(k), instead of the unnormalized steeringvector {circumflex over (u)}*_(ap,m)(k). For an implementation ofprocessor 420 c in TX spatial processor 490, each subband processor 710would receive either (1) the steering vector {circumflex over(v)}_(ut,m)(k) for each wideband eigenmode used for the steeredreference, for the spatial multiplexing mode, or (2) the steering vector{tilde over (v)}_(at)(k) for the beam-steering mode. If subbandmultiplexing is used for the steered reference, then steering vectorsfor multiple wideband eigenmodes may be used for multiple disjoint setsof subbands, as described above.

FIG. 7B shows a block diagram of an RX spatial processor 460 c that canprovide estimates of steering vectors and singular values based on areceived steered reference. Processor 460 c may be implemented within RXspatial processor 440 or 460 in FIG. 4, but for clarity is describedbelow for an implementation in RX spatial processor 460. Processor 460 cincludes a number of steered reference subband processors 750 a through750 k, one for each subband used for steered reference transmission.Each subband processor 750 receives a vector r(k) and a conjugated pilotsymbol p*(k) for the associated subband.

Within each subband processor 750, the four symbols in the receivedvector r(k) are multiplied with the conjugated pilot symbol p*(k) bycomplex multipliers 762 a through 762 d, respectively. The outputs ofmultipliers 762 a through 762 d are then accumulated for the duration ofthe steered reference transmission for each wideband eigenmode byaccumulators 764 a through 764 d, respectively. As shown in Table 9, thesteered reference may be sent for multiple wideband eigenmodes withinthe same steered reference transmission, in which case the accumulationis performed separately for each of these wideband eigenmodes. However,multiple steered reference symbols (which may be transmitted in one ormultiple steered reference transmissions) for any given widebandeigenmode may be accumulated to obtain a higher quality estimate.Accumulators 764 a through 764 d provide four elements which are theestimate of {circumflex over (v)}*_(ut,m)(k)σ_(m)(k) as shown inequation (13).

Since the eigenvectors have unit power, the singular value σ_(m)(k) foreach wideband eigenmode may be estimated based on the received power ofthe steered reference. A power calculation unit 766 receives the outputsof multipliers 762 a through 762 d and computes the received power ofthe steered reference, P_(m)(k), for each eigenmode of subband k. Thesingular value estimate {circumflex over (σ)}_(m)(k) is then equal tothe square root of the computed received power of the steered referencedivided by the magnitude of the pilot symbol (i.e., {circumflex over(σ)}_(m)(k)=√{square root over (P_(m)(k))}/|p(k)|), where

${P_{m}(k)} = {\sum\limits_{i = 1}^{N_{R}}{{r_{i}(k)}}^{2}}$

and r_(i)(k) is the symbol received on subband k of receive antenna i.

The outputs of accumulators 766 a through 766 d are then scaled by theinverse of the singular value estimate, {circumflex over (σ)}_(m) ⁻¹(k),by multipliers 768 a through 768 d, respectively, to provide an estimateof the steering vector for each eigenmode, {circumflex over(v)}*_(ut,m)(k)=[{circumflex over (v)}_(ut,1,m)(k) {circumflex over(v)}_(ut,2,m)(k) {circumflex over (v)}_(ut,3,m)(k) {circumflex over(v)}_(ut,4,m)(k)].

The processing for the steered reference for the beam-steering may beperformed in a similar manner. The processing for the steered referenceon the uplink may also be performed in similar manner to obtain anestimate of the steering vector for each eigenmode, {circumflex over(u)}_(ap,m)(k)=[û_(ap,1,m)(k) û_(ap,2,m)(k) û_(ap,3,m)(k)û_(ap,4,m)(k)].

The pilots described herein may be implemented by various means. Forexample, the processing for the various types of pilot at the accesspoint and the user terminal may be implemented in hardware, software, ora combination thereof. For a hardware implementation, the elements usedto process the pilots for transmission and/or reception may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, other electronic units designed to perform thefunctions described herein, or a combination thereof.

For a software implementation, some of the processing for the varioustypes of pilot (e.g., the spatial processing for a pilot transmissionand/or channel estimation based on the received pilot) may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory units 432 and 482 in FIG. 4) and executedby a processor (e.g., controllers 430 and 480). The memory unit may beimplemented within the processor or external to the processor, in whichcase it can be communicatively coupled to the processor via variousmeans as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method for wireless multiple-inputmultiple-output (MIMO) communication utilizing orthogonal frequencydivision multiplexing (OFDM) comprising: obtaining a set of pilotsymbols for each antenna in a plurality of antennas; obtaining anorthogonal sequence for each antenna in the plurality of antennas,wherein the plurality of antennas are assigned different orthogonalsequences; covering the set of pilot symbols for each antenna with theorthogonal sequence for the antenna to obtain a set of sequences ofcovered pilot symbols for the antenna, wherein the set of pilot symbolsare covered so as to reduce an amount of overhead associated with pilottransmission; processing the set of sequences of covered pilot symbolsfor each antenna to obtain a sequence of OFDM symbols for the antenna;and transmitting the plurality of sequences of OFDM symbols from theplurality of antennas.
 2. The method of claim 1, wherein the pilotsymbols correspond to one or more of a beacon pilot, a MIMO pilot, asteered pilot and a carrier pilot.
 3. The method of claim 1, wherein thepilot symbols correspond to a MIMO pilot and wherein covering the set ofpilot symbols comprises covering the set of pilot symbols using anorthogonal code corresponding to one or more of an M-sequence or atime-shifted version of an M-sequence.
 4. The method of claim 1, whereinthe pilot symbols correspond to a MIMO pilot and wherein covering theset of pilot symbols comprises covering the set of pilot symbols using aplurality of Walsh sequences scaled by a complex multipliers usingcomplex correction factors.
 5. The method of claim 1, wherein the set ofpilot symbols are for transmission on a set of subbands and whereintransmitting the plurality of sequences of symbols comprisestransmitting a plurality of pilot sequences on corresponding principaleigenmodes of respective subbands.
 6. The method of claim 5, wherein thepilot symbols correspond to a carrier pilot and wherein transmitting theplurality of sequences of symbols comprises transmitting the carrierpilot on the principal eigenmodes while using a steering vector forspatial processing.
 7. The method of claim 1, wherein the pilot symbolscorrespond to a steered pilot and wherein transmitting the plurality ofsequences of symbols comprises: obtaining a set of steered referencepulses for each of a plurality of eigenmodes; and transmitting the setsof steered reference pulses using time-domain multiplexing.
 8. Themethod of claim 1, wherein the pilot symbols correspond to a steeredpilot and wherein transmitting the plurality of sequences of symbolscomprises: isolating a single multipath component for a widebandchannel; processing the multipath component as a narrowband channel toobtain a single steering vector for the multipath component for each ofa plurality of eigenmodes; and transmitting the steering vector.
 9. Themethod of claim 1, wherein the pilot symbols correspond to a beaconpilot and a MIMO pilot and wherein transmitting the plurality ofsequences of symbols comprises: transmitting symbols corresponding tothe beacon pilot with information sufficient to permit a user terminalto achieve timing and frequency acquisition; and transmitting symbolscorresponding to the MIMO pilot with information sufficient to permit auser terminal to obtain one or more of (a) an estimate of a downlinkMIMO channel, (b) a steering vector for an uplink transmission and (3) amatched filter for a downlink transmission.
 10. The method of claim 1,further comprising: receiving MIMO pilot signals from a user terminal;and estimating a calibrated uplink channel response from the receivedMIMO pilot signals.
 11. The method of claim 1, wherein transmitting theplurality of sequences of symbols comprises: transmitting data on aplurality of wideband eigenmodes in accordance with spatialmultiplexing; and transmitting data on a principal wideband eigenmode inaccordance with one or more of beam-steering or beam-forming.
 12. Themethod of claim 1, wherein the pilot symbols correspond to a steeredreference and wherein transmitting the plurality of sequences of symbolscomprises transmitting a set of symbols on one wideband eigenmode in agiven symbol period while performing spatial processing with a set ofeigenvectors for that wideband eigenmode.
 13. The method of claim 1,wherein the pilot symbols correspond to a steered reference and whereintransmitting the plurality of sequences of symbols comprisestransmitting multiple sets of the symbols on multiple widebandeigenmodes in a same symbol period by performing spatial processing withmultiple sets of eigenvectors for the wideband eigenmodes.
 14. Themethod of claim 1, wherein the pilot symbols correspond to a steeredpilot and wherein transmitting the plurality of sequences of symbolscomprises transmitting a steered pilot generated using a set ofnormalized eigenvectors for a principal wideband eigenmode withinformation sufficient to permit a user terminal to derive a matchedfilter for a beam-steering mode.
 15. The method of claim 14, wherein thesteered pilot is transmitted in one or more of a preamble or a pilotportion of a forward channel protocol data unit.
 16. The method of claim1, wherein the set of pilot symbols is for transmission on a set ofsubbands and wherein a number of subbands used in each set is selectedbased on a signal to noise ratio (SNR) of a wideband eigenmodeassociated with the set.
 17. The method of claim 16, wherein subbandmultiplexing is used to reduce an amount of overhead needed to transmita steered pilot.
 18. The method of claim 1, wherein the pilot symbolscorrespond to a steered pilot and wherein the method further comprises:receiving a steered reference pilot; and deriving an estimate of aninput vector so that a mean square error between the received pilot andthe transmitted pilot symbols is minimized.
 19. The method of claim 1,wherein the pilot symbols correspond to a steered pilot and wherein themethod further comprises: receiving a steered reference pilot for onewideband eigenmode in any particular symbol period; and obtaining anestimate of one eigenvector for each subband of that wideband eigenmodefor use in one or more of matched filtering and spatial processing. 20.The method of claim 1, wherein the pilot symbols correspond to a carrierpilot and wherein the method further comprises selectively resettingpilot sequences for each of a plurality of transport channels fortransmission.
 21. A device for wireless multiple-input multiple-output(MIMO) communication utilizing orthogonal frequency divisionmultiplexing (OFDM) comprising: a plurality of antennas; a processingcircuit configured to obtain a set of pilot symbols for each antenna inthe plurality of antennas, obtain an orthogonal sequence for eachantenna in the plurality of antennas, wherein the plurality of antennasare assigned different orthogonal sequences, cover the set of pilotsymbols for each antenna with the orthogonal sequence for the antenna toobtain a set of sequences of covered pilot symbols for the antenna,wherein the set of pilot symbols are covered so as to reduce an amountof overhead associated with pilot transmission, and process the set ofsequences of covered pilot symbols for each antenna to obtain a sequenceof OFDM symbols for the antenna; and a transmitter configured totransmit the plurality of sequences of OFDM symbols from the pluralityof antennas.
 22. The device of claim 21, wherein the pilot symbolscorrespond to one or more of a beacon pilot, a MIMO pilot, a steeredpilot and a carrier pilot.
 23. The device of claim 21, wherein the pilotsymbols correspond to a MIMO pilot and wherein the processing circuit isconfigured to cover the set of pilot symbols so as to reduce an amountof overhead associated with pilot transmission by covering the set ofpilot symbols using an orthogonal code corresponding to one or more ofan M-sequence or a time-shifted version of an M-sequence.
 24. The deviceof claim 21, wherein the pilot symbols correspond to a MIMO pilot andwherein the processing circuit is configured to cover the set of pilotsymbols so as to reduce an amount of overhead associated with pilottransmission by covering the set of pilot symbols using a plurality ofWalsh sequences scaled by a complex multipliers using complex correctionfactors.
 25. The device of claim 21, wherein the set of pilot symbolsare for transmission on a set of subbands and wherein the transmitter isconfigured to transmit a plurality of pilot sequences on correspondingprincipal eigenmodes of respective subbands.
 26. The device of claim 25,wherein the pilot symbols correspond to a carrier pilot and wherein thetransmitter is configured to transmit the carrier pilot on the principaleigenmodes while using a steering vector for spatial processing.
 27. Thedevice of claim 21, wherein the pilot symbols correspond to a steeredpilot and wherein the transmitter is configured to: obtain a set ofsteered reference pulses for each of a plurality of eigenmodes; andtransmit the sets of steered reference pulses using time-domainmultiplexing.
 28. The device of claim 21, wherein the pilot symbolscorrespond to a steered pilot and wherein the transmitter is configuredto: isolate a single multipath component for a wideband channel; processthe multipath component as a narrowband channel to obtain a singlesteering vector for the multipath component for each of a plurality ofeigenmodes; and transmit the steering vector.
 29. The device of claim21, wherein the pilot symbols correspond to a beacon pilot and a MIMOpilot and wherein the transmitter is configured to: transmit symbolscorresponding to the beacon pilot with information sufficient to permita user terminal to achieve timing and frequency acquisition; andtransmit symbols corresponding to the MIMO pilot with informationsufficient to permit a user terminal to obtain one or more of (a) anestimate of a downlink MIMO channel, (b) a steering vector for an uplinktransmission and (3) a matched filter for a downlink transmission. 30.The device of claim 21, further comprising a receiver configured toreceive MIMO pilot signals from a user terminal and estimate acalibrated uplink channel response from the received MIMO pilot signals.31. The device of claim 21, wherein the transmitter is configured to:transmit data on a plurality of wideband eigenmodes in accordance withspatial multiplexing; and transmit data on a principal widebandeigenmode in accordance with one or more of beam-steering orbeam-forming.
 32. The device of claim 21, wherein the pilot symbolscorrespond to a steered reference and wherein the transmitter isconfigured to transmit a set of symbols on one wideband eigenmode in agiven symbol period while performing spatial processing with a set ofeigenvectors for that wideband eigenmode.
 33. The device of claim 21,wherein the pilot symbols correspond to a steered reference and whereinthe transmitter is configured to transmit multiple sets of the symbolson multiple wideband eigenmodes in a same symbol period by performingspatial processing with multiple sets of eigenvectors for the widebandeigenmodes.
 34. The device of claim 21, wherein the pilot symbolscorrespond to a steered pilot and wherein the transmitter is configuredto transmit a steered pilot generated using a set of normalizedeigenvectors for a principal wideband eigenmode with informationsufficient to permit a user terminal to derive a matched filter for abeam-steering mode.
 35. The device of claim 34, wherein the transmitteris configured to transmit the steered pilot in one or more of a preambleor a pilot portion of a forward channel protocol data unit.
 36. Thedevice of claim 21, wherein the set of pilot symbols is for transmissionon a set of subbands and wherein the transmitter is configured to selecta number of subbands used in each set based on a signal to noise ratio(SNR) of a wideband eigenmode associated with the set.
 37. The device ofclaim 36, wherein the transmitter is configured to use subbandmultiplexing to reduce an amount of overhead needed to transmit asteered pilot.
 38. The device of claim 21, wherein the pilot symbolscorrespond to a steered pilot and wherein the device further comprises:a receiver configured to receive a steered reference pilot; and whereinthe processing circuit is further configured to derive an estimate of aninput vector so that a mean square error between the received pilot andthe transmitted pilot symbols is minimized.
 39. The device of claim 21,wherein the pilot symbols correspond to a steered pilot and wherein thedevice further comprises: a receiver configured to receive a steeredreference pilot for one wideband eigenmode in any particular symbolperiod; and wherein the processing circuit is further configured toobtain an estimate of one eigenvector for each subband of that widebandeigenmode for use in one or more of matched filtering and spatialprocessing.
 40. The device of claim 21, wherein the pilot symbolscorrespond to a carrier pilot and wherein the processing circuit isfurther configured to selectively reset pilot sequences for each of aplurality of transport channels for transmission.
 41. A device forwireless multiple-input multiple-output (MIMO) communication utilizingorthogonal frequency division multiplexing (OFDM) comprising: means forobtaining a set of pilot symbols for each antenna in a plurality ofantennas; means for obtaining an orthogonal sequence for each antenna inthe plurality of antennas, wherein the plurality of antennas areassigned different orthogonal sequences; means for covering the set ofpilot symbols for each antenna with the orthogonal sequence for theantenna to obtain a set of sequences of covered pilot symbols for theantenna, wherein the set of pilot symbols are covered so as to reduce anamount of overhead associated with pilot transmission; means forprocessing the set of sequences of covered pilot symbols for eachantenna to obtain a sequence of OFDM symbols for the antenna; and meansfor transmitting the plurality of sequences of OFDM symbols from theplurality of antennas.
 42. A machine-readable storage medium for usewith a wireless multiple-input multiple-output (MIMO) communicationsystem utilizing orthogonal frequency division multiplexing (OFDM), themachine-readable storage medium having one or more instructions whichwhen executed by at least one processing circuit causes the at least oneprocessing circuit to: obtain a set of pilot symbols for each antenna ina plurality of antennas; obtain an orthogonal sequence for each antennain the plurality of antennas, wherein the plurality of antennas areassigned different orthogonal sequences; cover the set of pilot symbolsfor each antenna with the orthogonal sequence for the antenna to obtaina set of sequences of covered pilot symbols for the antenna, wherein theset of pilot symbols are covered so as to reduce an amount of overheadassociated with pilot transmission; process the set of sequences ofcovered pilot symbols for each antenna to obtain a sequence of OFDMsymbols for the antenna; and transmit the plurality of sequences of OFDMsymbols from the plurality of antennas.